Should my in-laws be invited to my sister's wedding?

There are a number of factors that might help HER decide whether YOUR in-laws should be invited to your sister's wedding.

First, have your in-laws been to social functions with your sister in the past? Do they come over to YOUR home at the same time?

Second, when they've met in the past, have they been cordial? Do they talk nice or are they mortal enemies?

Third, (and actually the only real deciding factor) what does your sister want? She should be making that decision; after all, the reception is typically about the bride more than anyone else. If she wants your in-laws to attend, she should decide and nobody should attempt to influence her.

It's her special day, so if you've made a suggestion, let it go. She'll either agree or not.