Ugh, I'm so sorry. Holidays can be rough. How has DH been? Don't let him give you grief if you decide to decline any blended activities. Acknowledging that "this sucks" is okay and meltdowns are okay too. No need to hold it together for anyone. Everything can be on your own terms. Maybe DH can put up the tree? If you aren't feeling in the holiday spirit, he should be able to pick up the slack for that when SD comes. Don't pressure yourself to "put on a face". 

Firstly, im so sorry. That's alot of loss of some very close loved ones and so quickly.

i will say what you are experiencing now, especially a few months post their passings, is normal. As a child i had cancer and therefore a lot of friends with cancer, by the time i was 8 i had been to 4 funerals. By about 20 I reckon about 15, so I have always felt rather "seasoned" with handling death.

At age 26, my mum was 47 and phased away from cancer within a couple of years of her diagnosis. We were very close. It absolutely through. I felt how you did. It was on my mind, things people had said to me that I had let slide during her battle and post her passing i would ruminate on and most of all my first emotion to a lot of things was anger. I found it especially hard at work and in step life. I was overwhelmed with "everyone around me's life hasn't changed, but mine has and I sit here completely different, and they just go on none the wiser" and also with every little (or big) annoyance or irritation I would go straight to "my mum has died I don't have time for this shit"....

This lasted a few months, but the only thing I found that helped me was talking to someone. A completely unknown, unbiased person to my situation. I made an appointment with a psychologist who was a lovely older woman who specialised in grief on her resume and I would just talk. I found saying some of the things out loud that you push down about being upset, about not having them here, about all the people around you that don't get it and never will, just allows you to release it. I think I went about 4 or 5 times and I can't say she necessarily gave me any advice but just validated my feelings and sat there acknowledging that certain things weren't ok and were unfair and didn't try and "solve" it. 
Outside of this, I also found writing to my mum or someone who had been pissing me off and really telling them how I feel, like really just things you wouldn't verbalise and then throwing it away helped.  Maybe it's something to do with unburdening yourself of all those ruminating thoughts. 

None of this will take the loss away, but it does maybe start the process for you to move past the anger portion. 
 I'm not sure if any of this helps but also remember it has only been a couple of months. We try and rush through these emotions cause they are so unsettling and not our norm, but it wil take time (i used to hate hearing that, but it does) so please be kind to yourself and if you do lose it as someone, give yourself a pass xx

I am so sorry you lost your sister and mother and so close together, must be awful. Completely understandable having no patience with the shennanigans of SKIDs.  I remember back in 2010, over Xmas my mother was dying and I had to put up with SDs getting drunk and vomiting on the carpet in the living room.  I nearly lost it, and would have been happy never to see them again.  I would suggest making sure SD knows that she is to be on her best behaviour, that you have had enormous losses and are on a short fuse.  I wish you well. 

I can't imagine two losses so close to one another and I don't blame you one bit for not being full of the festive spirit.  Shoot.. I don't feel festive.. but it's nothing to do with any specific loss.. I have always struggled with the holiday because it is so hyped with anticipation and always seemed to fall flat for me growing up.  My dad was a kind of morose guy himself around the holidays.. so maybe his unpredictable moods made me just always be on edge.

Now, I try to always be out of town.. somewhere warm for the holidays where I can have as little focus on it all.. 

I am sorry you are having to feel like you have to put on some happy face when you just don't feel up to it.  that has to suck.

I guess I would start by saying.. be kind to yourself.. give yourself breaks.. do small acts of kindness for yourself.  do things that have some potential for bringing you joy.. even if it is just stopping by your fav bakery to buy yourself one cupcake..   If you are able.. maybe participating in some kind of angel tree gift donation.. to try to make someone else's holiday bright might make you feel you have done something nice for others.. and give you a little lift.  Let your SO do the heavy lifting with his child.. he has to understand you are struggling and not put too much pressure for you to perform.

Take care of yourself.. you deserve it.

Can you afford to run away for some self care? Hotel? Airbnb? Friend's house? A change of venue ALWAYS helps me and I would not, cannot fake "I'm okay." I wear my feelings on my sleeve.

IDGAF what anyone else thinks. If I went through what you've been through, I would probably lock myself in somewhere, eat/drink everything I wanted to, and sleep/watch dumb TV for a week or more.

You must give yourself permission to do this.

Is there a place that meant a lot to you and your sister? You and your mom? A place you talked about going? GO - do it.

Anyone who says, "But you need to be here and do [A, B or C] because it's Christmas" can eff all of the way off.

If no one is taking care of you, the responsibility is yours. Take care of yourself in a way that makes sense to you.

I live about an hour from a monastery where they have 2 hermitages. They are two tiny houses in the woods available to anyone, regardless of your religious beliefs, to escape from the world. The places have no wifi or TVs. You can get a decent cell phone signal, but you can also choose to shut out the world. It's a reasonable fee - about the same as a night in a hotel.

(((hugs)))

I am the same boat.... stressed out, on the verge of exploding all the time not thankful, not in the Christmas spirit etc... what you're going through is miles harder than me just dealing with LI though. I don't know how to stop from exploding, if you figure it out let me know! Don't feel obligated to do anything you don't want to this year. 
 

I  can't imagine going through what you have.  How awful, I'm sorry.

Around this time each year, I see some of the local churches hosting "Blue Christmas" services for those who have suffered losses.  I've  never been to one yet but it might help if something like this is available in your area.

Otherwise, I'd do the bare minimum, or only exactly what you are up for.  December 26, the day of relief, is only 16 days away.

Damn, I'm so sorry for your losses. Don't do anything you don't want to do. Especially for ingrates. Focus your time on yourself and on those you still have that you care about and who care about you. 

I am so sorry for your loss.  Invest in embracing what you are feeling and in your grief.  Enjoy your dad and your sister.

If  you need it, spend Christmas with them or with friends and let DH and SD navigate their holiday without your having to suffer through it.

((((Hugs))))

My deepest sympathy for your losses. {{{hugs}}}

​​​Right now, you need to put yourself first. Let your DH handle his daughter. He can entertain her, feed her, whatever has to be done. That all needs to be on his plate. 

I also vote for you to get away for a day or three, if that's possible. If not, once again, all SD stuff needs to be on her father's plate. If he wants any entertaining, it can be done without you. You're grieving and need time to heal.

I'm so very sorry for your losses. I understand your feeling "on the edge." We once had a visitor in our home following a loss. I felt like I was going to lose it because of the person acting like a sociopath. 

I would avoid SD as much as possible. Make appointments for yourself: Go get a massage; a mani/pedi. Go to a yoga/meditation class. Go to a movie with friends. Go out and eat. Watch movies in a private room. Tell DH to decorate. Tell DH to take SD out for the day.  And then the next day. And, yea, I'm with the others who have mentioned renting an Airbnb, a cabin in the woods or the like. If you have to lie and say it's because you are sick, do so. As Rags likes to say, take care of you!