It's a shame that my kids and my nephews can't stand each other especially when I am very close with my brother

I have two boys, 6 and 9 and two nephews who are 7 and 10. My brother and I are very close and we wanted our kids to be best friends instead they generally dislike each other. My kids tend to be a bit more reserved whereas my nephews tend to be a bit more wild. I also thought they would be more sensitive towards each other but they are nasty to each other. I asked them if they are that mean to kids at school and they said no. I asked then why are they so mean to each other and their response was because they don't like each other. via /r/Parenting https://ift.tt/2wBAprp