I can't imagine why your BF would have a problem with this. <br />
Perhaps offer to let him sit this one out, if he's uncomfortable, and go without him.<br />
It is certainly not wrong or disrespectful on your part.
Then go without him. If he makes a huge deal out of this reconsider the relationship. You were related to these people before you knew him. Anyone reasonable and emotionally-mature would understand that.
its not like there is bad blood between you and your former sil just talk to your bf hes just feeling insecure about the whole thing I would go if it was me