My DB and SIL have a DS same age as my DD (6). They regularly ask me and other family members to look after him overnight (which always turns into full weekends) when they go on weekends away together, or on work-related trips. In the past I didn't mind too much as DD really loves her cousin, though I did have some concerns about how upset he used to get about his regular sleepovers with various family members (not anymore though). However, I am finding it more and more difficult. He is extremely competitive with DD (though not with other cousins), and becomes very sulky and aggressive if he doesn't beat her at everything. He will ask her questions that he thinks she can't answer, just so that he can laugh if she gets it wrong (sulks if she gets it right), and when he thinks no-one is looking he pushes or pinches her. I also find it really annoying that he is so clearly delighted if he manages to upset her, if she hurts herself or if she gets told off. TBH I would rather we saw a lot less of him, but DD loves him and is always making excuses for his behaviour. Have tried talking to DB and SIL, as well as other family members but they all think think I am overreacting and are of the view that my DD is far too clever to let her cousin get the better of her and that he is just being a typical boy. I would really like them to have a nice relationship as DD has no siblings, but he just doesn't seem to feel kindly towards her at all. DH thinks he is jealous as her life is more secure than his. Am I being too sensitive?