Wow, that sounds like not only an annoying situation, but one that is potentially dangerous to the two toddlers in the house. You are not wrong to expect the 8-year old to respect you and your child. She sounds like a handful.
A couple of thoughts...(and you may have already considered this!)
Ask your husband to have a conversation with his sister, the mother of the child, explaining that the 8-year old has been told repeatedly not to roughhouse with the babies and that he is concerned for their safety.
Arrive late, eat quickly, go home.
Keep your baby in your arms the whole time- which would be nearly impossible for a toddler to handle, especially if there are other kids around. But, if you only stay for 45 minutes, this might be a viable solution. If someone asks, just say that you are worried with all of the commotion that the baby will get knocked over.
Your husband goes to dinner, you stay home with the baby. He can let the adults know the reason. He can have a conversation with the 8-year old, saying "Auntie worries that you will hurt our baby because you are so rough with her and you have not listened when Auntie asks you to stop." Then, have your husband and the 8-year old come up with a plan for how to manage herself around the babies. After a few weeks of practice, if she can manage herself around the other two cousins, perhaps return for another try.
If it was me and I was pregnant as you are, I would pick the stay-home option until things settle down.