re: Rock (wife) between (me) Hard place (my family)

Your sister said something about your wife that was unwarranted per you. When did this happen? Why did your sister say it and to whom did she say it? What did you do about it?

If your sister isn't nice to your wife, then you might explain to her that you are married, you love your wife, she was wrong and needs to apologize or you will not be attending her functions.

If your sister "really wants US to attend", then she needs to do what she needs to do to fix this.