Hi Remy - I have a few pretty simple thoughts... First of all, these things usually bother us more than they bother the kids.
And - you will not be able to change who your in-laws are.
Simple concepts, hard to internalize and create a different energy that you can bring to the relationships.
Your kids are not aware of who visits who and when. They are busy doing their own thing. Because you know, and you are "reading" the message from your framework, you are the one being bothered.
See the connection here? The only person you can do anything about is you - the only attitude you can change is yours.
SO - keep your kids busy with their lives and their friends. Get them together with their cousins when you can and when it's convenient.
Change the energy you bring to the family relationship. When you are with your in-laws, be cheerful and stupid (to quote Dr. Joy Brown.)
Enjoy what you can. You will find that once you adjust your attitude towards your in-laws and your relationship, things will change... Who knows? Maybe they will see changes in you and make changes in themselves!
Slow down, mellow out, enjoy your children - that's your focus.