I'm thinking of excluding one of my sisters, but taking the other on vacation.
Why do you want to exclude your sister?
One sister is more mature than the other.
You shouldn't exclude your siblings. Maybe you can make sure to balance your time between the two of them. If not, it might cause tension and competition between the two of them which can harm your relationships.
My parents separating kept us apart, which is why I don't know how to handle it now.
You can just not exclude anyone, that is all. Be kind to all people, especially family. You won't regret it later.