So, hot take here, but I wouldn't say that this comment by the parents rises to the level of abuse, or is a malicious act by the parents to manipulate op. I had 2 younger sisters, and after the youngest passed away my mom would say things like "you need to be nice to your sister, she's the only one you have." To be fair, she said this to both of us, often because we were arguing, but it wasn't to manipulate us into being well behaved. She did it to remind us that it was important to forgive each other, and let go of resentment over petty squabbles. Op's parents, sister, and BIL are 100% in the wrong, but I wouldn't say that their intent is to guilt trip Op, as much as to promote family harmony, and remind Op about the consequences of becoming distant from her sister. Remember that they went through a trauma too, and their actions are informed by that trauma. Intent matters as much as actions. Remember that it's 'us' against the problem. 'We' need Op to enjoy the family get together, bond with their sister, and for all the young children at the party to be safely looked after. If we lay it out like that, then we can start to tackle the problems together.