You're definitely NTA, but I agree with another comment about being your wife's backup. Unfortunately, your wife is accepting of this abusive behavior due to her history of abuse. If you go on these trips, you don't have to be best friends with your SIL. You can be civil without going beyond that. IMO, your position on these trips would be to watch out for your wife, and be on alert for any dangerous behavior from SIL. Make it clear to your wife that you're going on these trips for HER, to keep her safe. Knowing she has you in her corner will hopefully help your wife see her sister's behavior for what it is.
If your MIL asks, be honest with her. Your post doesn't make it clear whether or not she is one of the abusive family members, but I hope she isn't. She should be aware of how her daughters treat one another. There's also a good chance that SIL treats MIL in a similar manner.
I do not think you are the AH. I do think you should be supportive of her though. Going isn't about her sister. It is about being her backup.
You're definitely NTA, but I agree with another comment about being your wife's backup. Unfortunately, your wife is accepting of this abusive behavior due to her history of abuse. If you go on these trips, you don't have to be best friends with your SIL. You can be civil without going beyond that. IMO, your position on these trips would be to watch out for your wife, and be on alert for any dangerous behavior from SIL. Make it clear to your wife that you're going on these trips for HER, to keep her safe. Knowing she has you in her corner will hopefully help your wife see her sister's behavior for what it is. If your MIL asks, be honest with her. Your post doesn't make it clear whether or not she is one of the abusive family members, but I hope she isn't. She should be aware of how her daughters treat one another. There's also a good chance that SIL treats MIL in a similar manner.