How does your DH feel about the sass and the lip?  If you've read on here, you know most of these issues start with the parent and there's not a lot we step-parents can do if the bio parent isn't responding.

Read around on Steptalk and you will see hundreds of cases like yours where guilty dad defends poor behavior and stepmom is evil for bringing up the topic.  You might want to look carefully at the Disengagement section of the Forums.  It's the last resort for many of us.

3.5 yrs into my marriage and 2 kids with hubby. They had disappeared and ended contact with their dad for 5.5 yrs.

they answered back and actively in fromt of their dad would answer back when i said very clearly "no don't do that to my kids because they will get very sick" etc and the little shits would say "its ok we'll give a little".

i answered "NOOOOOOOOOOO" and hubby would hear that and say "listen to your stepmum"

it wasn't till about 1.5 yrs later hubby was forced to address this with eldest sd. He reminded her that i was his wife and the mother of our 2 kids, therefore what i say goes!! There is no answering back and he reminded her she was his daughter, not the mother of our kids and therefore should no her place is below mine when it comes to our kids.

she sulked but biomum had elevated eldest sd to mother figure of youngest sd and so she thought she was level with me. I made it clear to my husband there would never be a day where i would ever have to justify myself to sd's or have them answer me back ever and if he was not gonna do anything about it, then this marriage would no longer continue.

np way do I tolerate answering back and toxic arseholes. The whole "oh but they're rebellious teens and their mum made them this way", nope, I don't give a shit what goes on in bio mums home. In our household and our family, this shit doesn't fly!! 

I agree with those who said you signed up to be his wife- that's all. Your vows were to each other, not to his kids. They have 2 parents, you aren't either of those parents. 

I also understand bonding more with your SS than SD. I have 2 SS's, a BioSon and a BioDaughter. GIRLS ARE HARD. Like.... REALLY HARD. Especially now as a teen. I am not prepared or even feel equipped some days to deal with my own daughter. She is mean now. She used to be the kindest, sweetest little girl. Then she turned 13 and this flip switched and holy crap. She is almost 15 now and starting to show moments of the girl that I have raised again, but man. She is tough. 

But going back to your real problem- It is your husband. Explain to him that you have no problem being "Fun Auntie" that you are there for the good stuff but you aren't going to fuss at them. You aren't going to follow up with homework. You aren't going to enforce chores. HE IS. And you will privately tell him when he needs to address something that is bothering you and HE WILL deal with it. You will make popcorn and set up movie night The end. Get on board or find someone else to be your door mat. 

I heard the "you signed up for this" just once and it wasn't about me - it was a conversation about the SF.

I put a stop to it right there and then.  I said "no, he didn't sign up for THIS and neither did I. What we signed up for is what we were lead to BELIEVE.  We believed that the kids had 2 parents, not 4.  We believed that you co-parented in a healthy way and did things for the best of your children.  We believed that you would both parent your children and teach them right from wrong, respect and common sense"  I told DH that I love him and am with him inspite of his children. 

Since DH is sensible he understood and the light clicked on.  DBDB not so much.  She is just living her life trying to destory DH's.

LOL. That is a guarrantee. You absolutely did NOT "sign up" to be dealing with bratty skids. There are a few different issues that you brought too light in this intro post.

1. SD is "crabby", and daddy-cakes does the monkee dance to appease her. She is doing this precisely to see the monkee dance. HIS behavior is encouraging HER behavior. He is effectively rewarding her behavior.

2. He gets defensive when you try to discuss this with you. You and he are a team, you and he need to be united, not divided. This will get worse over time, not better. Read on here, and as mentioned in other comments and other posts, the issue is primarily the parents. You cannot control what happens with the other household. all you can do is manage your own household. Is she the 14 year old or 11? She sounds like a teenager.

I got  lucky with my younger SD - shes generally a people pleaser and kind and respectful. Her sister on the other hand is a nightmare. She is 22, and I met her at 15. Luckily she moved far away.

Your husband is helping to create a literal monster. It does not bode well if you cannot discuss this with him.

I can only imagine how your SS feels. He's the target of SD's bad behavior, and his own father won't intervene. Talk about telling someone that he is not worthy of basic manners and civility must be crushing.

"You knew what you were getting into" is just code for "I'm too lazy to parent my children and how dare you call me out on it."

You are not required to go on outings with these brats.  Don't go unless/until DH reins them in.  Curl up with a good book or your favorite TV show.