Let's see if I am reading this right ...
1) You had 3 chins to begin with.
2) The youngest of the three passed away in April.
3) The one that passed away use to have rough looking fur on it's hips.
4) Now that it is gone, the other female is appearing to have acquired rough looking fur on her hip.
5) The male is the dominate one.
The stress of loosing a cagemate could cause either or both to start chewing their fur.
The cats could also be causing them some stress. You might want to make sure they always stay in different rooms. We are in the dog days of summer, which always seems to cause animals to become more aggressive.
If the apartment is getting to warm, that could stress them. That could also kill them. What does the temperature get to inside your apartment?
Fur chewing can also be a sign of something lacking in their diet.
Jo Ann