Is it normal that my sister is wanting to get close to my husband's family?

My sister and her husband trive on gossip. Gossiping is the only thing I talk about with my sister. We really have not much in common, other than the people we know. So whenever we catch up, it just turns into gossiping about the people we know. I don't mind gossiping, but not to an extent she likes. I avoid it, and never gossip about my husband's side of the family.

My sister has kids, and so does my husband's sister. My sister recently invited my husband's sister and her kids to a kids birthday party. I thought it was strange, because there were plenty of other kids and I know for a fact that she doesn't think very highly of my husband's sister...

What's worse is the number of facebook messages she sent in order to get the address so she could send the invite to.. WHen her messages were ignored, she looked up the address in white pages and sent off the invite anyway... Long story short... my husband's sister and kids went to the party. My brother in law was there and said to my sister in law that they should catch up, and she said ok.

So, the other day my sister tells me that they are planning on inviting my husbands sister, hubby and kids for a bbq in the not so distant future and that they will invite us too...

What worries me, is that when things turn sour between them, my husband and I will have to deal with the consequences, and will have the troubles of having to keep everything separate so as to avoid the awkwardness of them being in the same room.

Also, I know for a fact that my sister is going to try as much gossip out of them as possible, which is just morally wrong in my opinion to befriend someone for the sake of gossiping.

Is this normal? Or am I overreacting?