The thing that would concern me about making sure the cousins are aware of each other, as OP said she did, is that it is forcing it on them and almost like rubbing it in their faces that these people exist who are meant to mean something to them and they are missing out. That's different to casual references to the people. I wouldn't be on board with my MIL pushing cousins we have nothing to do with on my children, especially when so young. The kids probably don't know what to do with that information.
Unused rooms, heat storage and heat transfer
Good Morning Monday 29th June 2026
In the end the children suffer. 
