Do they love each other? 
I have 2 from the same litter (brother and sister) and one elderly male cat.
The 2 younger ones love the older cat but especially the boy. When old cat goes out, little boy cat cries. When he comes back in, little boy cat runs to him, headbutts him, snuggled up to him purring ...
Then elderly cat gets fed up and swipes him!
Does anyone else have this?