SD12 and the whole gaggle of in laws were at my house all weekend. 12 freaking people, I was also angry at my husband. 

I have 2 SILs, NiceSIL has a baby and toddler, InsecureSIL thinks she is SD12's peer even though she's 26 and acts like she's a poor damaged child of my husband's divorce. 

SD12 was playing with my daughter and NiceSILs daughter, both about a year old. She was being quite rough and making them dance to WAP. This song is very inappropriate and I wouldn't have let SD watch this video so obsessively if she was my kid but oh well that's not my problem. She handled NiceSIL'd daughter very roughly and when the baby began to fuss about it SD12 sort of flung her towards her mom expecting her to be able to catch herself and walk. This baby has only begun to walk in the last couple weeks! NiceSIL was clearly very uncomfortable and gave me a pleading look to do something as insecureSIL cackled along with SD12. I said "Hey they are people not toys" and picked up my daughter who had started crying and looking for me. InsecureSIL gave SD a look like "whoa this B is ruining our fun." 

My DH wasn't there but he was fine with how I handled it. He has been a lot better at setting limits and not tolerating crap from SD since we had the baby. The problem is his family. They either act like they are afraid to reprimand her or they encourage her bad behavior. NiceSIL was afraid to say anyting to sd12 even when she was being way too rough with her baby daughter. Meanwhile InsecureSIL was laughing about it and the two of them seem to always try to be pushing that they have some sort of special immune status. They don't have to pick up after themselves they don't have consequences for their actions and they expect everything to be bought for them without question. My mil doesn't help the situation because she was yelling at one of the older cousins to pick up after SD 12. It is out of the question for any of them to ask her to pick up after herself!

My husband has been a lot better about having reasonable expectations for SD he even made her do dishes or the weekend. of course after doing that she began to avoid him and stuck very close with insecureSIL.

Oh and just like every time we see her InsecureSIL tried to "trick" DH into saying SD12 is his favorite child. She does this literally every time we see her.