hmmm that is a tricky one. it's most likely because he wasnt with his mum long enough and hasnt learnt proper social behaviour. all the weeing and pooing will probably be turning kitkat off a bit, and if Leo is a bit socially immature he is probably a bit of an annoying little brother to her lol.

my 2 cents would be to keep him in a small room for a few days. this is recommended whenever a new cat is brought into a home in any case to get them used to the new digs. have his litter in that room and he will get that he has to do it all in there as the he cannot hide from the smell. put his food and water in there too. i know it sounds kinda like a banishment, but it will give kitkat some time-out from him as well as some reassurance. we have done this for our new cats when they have come home and they all love us lol!
another idea is to visit your local vet and ask them for tips. but in my experience it kinda sounds like a case of 'too much too soon' and he doesnt feel settled enough to know where 'base' is ITMS. for a room we have used a number of places like the laundry, bedroom etc. i found the bathroom too tricky as we were in and out too often...

hth babe. but i reckon this could be the trick...and another thing, when he does come out for little visits during the day, if he makes a mess, pop him back into the litter in his room and close the door. they get the hint pretty quick IMO lol!