You defend yourself as brutally as necessary to bare the ass of the toxic SD and confront your IL clan.  Clearly communicate your expectations of support and defense to your husband and make it crystal clear that he will have your back as  you shred his toxic gene pool if they so much as twitch in the wrong direction.

Zero tolerance works.  Try it.

IMHO of course.

In our life......

Before we travel to SpermLand to visit with my IL clan my wife and I synch on all of the most recent IL clan drama and put contingency plans in place for what we believe will be the most likely drama scenarios.  Rarely are our forecasts accurate but when we are prepared with some basic contingency plans we can adapt quickly to confront any bullshit... and there is always toothless moron bullshit to confront.  Just maybe not the bullshit we forecast.

When you do not react to their aggression and just sit there and smile; they wonder what you are thinking, what you are going to say, and what you will do next.  I love to see them squirm just wondering.  Silence is an excellent weapon.  I have never responded to my wicked SD57 nor her daughter SGD34.  They are vicious women and I will not respond to them, thus giving them what they want, a fight.  I just sit there and smile and sometimes just glare at them, without a word said. 

My SD57 even had a melt down and wrote me a 2 page hate email.  I printed it out and handed it to DH to read.  He was shocked and took it up with SD57.  What DH said to her, I do not know, I didn't ask.  I am totally disengaged and they are blocked from all of my social media, they do not know my phone number, email address, nothing!  It has made both of them furious, because I ignore them.  I love the peace.  They are now trying to be nice to me.  It's only to pull me back into their vicious circle, NOT happening.  I do not go to their family gatherings and they are not invited to our home for social events.  They call before they come to visit DH.  I do not hostess them during those visits.  It is working well for me.

With that said, I do prepare myself for the verbal attacks; it helps me not respond.  These women cannot help themselves with the nasty remarks, no matter if they are around me for only a few minutes.  Their mouths are in neutral.

(((hugs)))

It depends on what you mean by Win.  If you are thinking that you will be believed by everyone and gain the upper hand with your SKs, you are correct in stating that you can't win.  That all too familiar saying, blood is thicker than water, holds very true, and especially true in step-situations.

Because of the way our culture looks at SPs, you are starting out at a huge disadvange to begin with.  You have BM (or at least her memory) and bio-dad you are dealing with.  You have COD/SKs.  You have in-laws that may have already been put in a position to choose sides before you even entered the picture.  And, you have society's expectations that somehow you are supposed to make everything right for everyone pretty much all on your own, otherwise there is clearly something wrong with you.  Unless you at least have both your DH and BM to smooth the way for you, you pretty much can't win.  Ever.

Sounds dismal, but this is where you get to make decisions for yourself, alone.  I always say a SM has to do what works for her, because no sooner does she try or do something, then there will be someone there telling her whatever it is is wrong.  You see, in our society, it is also accepted that pretty much anyone OTHER than a SM knows how to be a SM.

So, do what works for you.  Disenage, speak with DH about it and agree that he is to take some sort of action, bring it up on your own with the parties involved to "discuss."  Tell them you are married to DH and will be at least respected as his wife, otherwise they can go to H-.  And, so on.  Do what works for you, because you are already at a huge disadvanage.  The only thing you have to accept is that you can say "No," even to your own DH, and that you will be considered Evil SM no matter what you do.  So. . . do what works for you and what keeps the most peace for you (and your marriage).  Take care, and best of luck to you!

I hope there are more pockets of support in the inlaw family than you might suppose. Definitely defend yourself if questioned at the next event. Keep an eye open for family members who remain silent and don't attack. They may be thinking more in your favor.

If you are this stressed, it is time to stop being the family punching bag.  That is all you (many of us), are--no matter how hard we tried to be included.  For whatever reasons (and they were there long before us); daddeeee is not  supposed to bring in a wife. I am not certain it is related to any other issue most of the time, either.  They want nobody else to have daddeee's attention.

If that is the case, your only peace is away from it.