Sure.
I am dating someone but my brother is dating my girlfriend's sister. Is that allowed? Thankz
Yes it is my grandmother and her husband got married and her sister and his brother got married also its not insest or weird
Yep. My dad's brothers married sisters.
The thing about incest is that it's bad because of the genetic consequences. Being that you still aren't related to the girl you're dating, nor your brother to his girlfriend, it's not incest.
I don't see why it would be an issue at all.
It's sweet actually IF all four people can get along.
Which one?
You are dating "someone" or that you brother is dating you Girlfriend's sister?
Both seem highly permissible unless you have something against it and you;ve just not communicated that to us yet . . .