Possibly because they don't have any actual biological relation to her? I hope they aren't mean to her, or exclude her in a way that's obvious to her, as that could be hurtful to her. On the other hand, I do think it's hard for a lot--maybe even most--people to feel as close to non-biological family relations as they do their own flesh and blood. My brother's wife has a son from her previous marriage, and I freely admit I don't feel the same connection with him as I do with my brother's child biological child he has with his wife. It may sound awful, but it's honest. I would never be cruel to her son though--he's certainly always invited to the family get-togethers and holiday events of course.
You might try posting this in the "relationships" category for more answers.
at 10:00 AM on Aug. 27, 2009