Is this possible and how easy is it?

I'm asking on behalf of my mother in law who adopted two kittens at the same time as us (different breeds).

Unfortunetly she lost the boy kitten last week and the girl is said to be pining alot. Sue (MIL) has been onto another breeder who has two boy kittens of a similar age and she would really like to adopt them to come and live with her female kitten.

I said i'd try and find out some advice on this and whether it could work - any advice is appreciated

Thank you