Hi Kate
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I would say that it sounds like the youngster would be much better off in your care than that of the person who is caring for her now. Again, I won't lecture about her being un-neutered but at least you are willing to do this and care for her properly! I only live in a one bedroomed flat and my cat is mostly indoors and I got her when she was a lively 5 year old and she's absolutely fine! Just lots of toys and playing sessions and that's fine.
If she's very lively, it might not be wise to get something that is quite elderly as the youngster might pester the older one and may upset it by wanting to play all the time. On the other hand, when my cat wants a break (even just from me) she curls herself up in the airing cupboard so as long as you're aware of the difference in ages and the fact that the older one may need a break once in a while i'm sure you'll be fine.
If you end up not taking the youngster, can you please make sure that the owner gets in touch with either Cats Protection or the RSPCA so that the cat can be neutered. She sounds like the kind of person who may panic and put the cat up on the internet for adoption where people specifically scout for un-neutered females that they can use to breed in kitten farms. If the cat produces kittens it is unlikely that a charity will be able to take them in immediately and if she doesn't like a litter, boy is she not going to like kitten who just poo and wee all the time!
Sorry for the long reply, just reading about how the cat is being kept makes me very angry. Do you know if the other cat is neutered?
Jen