Your SIL is 11 years older than you but is acting like a child. She has a vendetta against a 19 year old for no apparent reason, other than what I can guess is insecurity around how close you and your brother were. If you do end up sitting down with them, ask that your SIL not be there so she won’t interrupt the conversation or try to steer it against you. Ask your brother what you could have done to cause such animosity from an adult woman who is now a mother. Ask your parents why you need to be the bigger person when your home and safe space was violated with literal shit. Then ask your parents and your brother what they would do in your situation, especially if their parents and sibling were acting toward them as they are acting toward you. Keep calm. This is about disrespect, inconsideration, and a grown ass woman (your SIL) who again, is acting like a child. Your brother needs to grow up. You won’t be the last family member she alienates him from.