So mad!!

I may be wrong for feeling this way or I'm just letting it get to me but my brother in law had a baby with a girl who mentally is a baby herself other than the fact that she is really in mature and down righ naïve she is ok but what really gets me is there baby is 6 months old she has her head in her tablet more than 70% of the time and when my husband try's to tell her something like " jr probably wants on the floor to crawl around" she looks him dead in the face and says how do you know you don't have kids! I mean seriously wtf my hubby and I have been ttc for a few years and it hasn't been easy on us her comments like that one or are you pregnant yet really get to me and I just want to scream at her. I love jr to death and my hubby is just trying to be helpful not a dick and she makes it really hard to like her. Idk what to do I no I'm gonna loose it one of these days on her but I just don't no how to handle someone like her. I mean she really is a child and is unwilling to grow up. Help?