I agree with the PPs. I think you need to stay out of it.
I had a situation like this, recently. My coworker and I have a *very* sarcastic relationship, and a brother/sister kind of jokey thing going on. I'm sure lots of people who don't know us would cringe at the things we say to each other, but if we ever cross the line, we can be honest. I think it's happened twice in the 5 years we've worked together.
He posted a picture of his daughter, and my comment was something along the lines of how cute she was, despite his genes.
His dad sent me a PM on FB, asking what the hell I meant by that and how offended he was. Not only did I think Dad was ridiculous, but so did my coworker, who thought my comment was funny.
I now get to tell people the story about how I blocked my coworker's father, on FB.
If your aunt or mom is offended, great. They can remove it. But I think without knowing the background or relationship between your aunt and this person, it's not your place to get involved.