I'd worry about the sister. Did she tell you about Mark's comment to her? How thoughtful of her, just what you want to hear, NOT! It doesn't sound like a very positive family altogether and I'd be a little leery of John, even, if he grew up with this crew.

If you aren't around Mark, I wouldn't worry about it but if you are going to be places where Mark and the mother, sister, rest of the family are; I wouldn't like it, would not feel welcome, no matter what John "said". How is it that he is around this wonderful family and/or doesn't speak up for you and/or that they're ALL talking about you behind your back when the other family members don't know you?