It sounds like you'll need to stop being a pushover for your sister. You are the mother: allow no more such pictures, or anything else that makes you uncomfortable for your son, no matter what other family members say.

This is a form of emotional blackmail. Basically, a temper tantrum.

Which is more important to you: keeping your sister out of a grudge or protecting your son? Because: I am guessing that, sooner or later, you are going to have to choose.

Be ready, and be strong.