First of all, you and your wife need to find a way to have more alone time together. It can be difficult once you have children, but it is healthy. Second, I don’t think it is unusual or unreasonable that your wife and her sister occasionally like to spend time together without their spouses. Just as a marital relationship is special, so is a sibling relationship. I’m not sure why you feel threatened by their bond.
It sounds like you and your wife spend a great deal of time together and have a close relationship. However, I disagree with you in stating that new relationships should be able to handle time apart, but it’s not acceptable once you have been together longer. I actually feel that it is typically the opposite. Most people in new relationships want to spend all their time together but as we mature and feel more secure in our relationship, time apart is quite normal for most couples. There needs to be an “I” and a “we” in a healthy marriage.
I would suggest that you plan a vacation — or at least a weekend away — alone with your wife, and work on making more friends or picking up a new hobby for the occasional times that your wife goes to see her sister.
All the best,
Dr. Holly Counts