I adopted 2 kittens, sisters, at 8 weeks old. they are now about 14 or 15 weeks old and one sister, she’s a longhair, is not cleaning her rear end after she uses the litter box, but her sister does. so we have to clean her with a baby wipe or something like that. is there anything to encourage her to clean herself? does her long hair get in the way? or is this just a kitten thing?