I started watching my neighbour's little girl twice a week a few months ago. I found it really hard at first, and DD definitely did not like sharing me. My situation may be easier because the girls are only 5 months apart in age and they do play together, but my point is it's much easier now that I've found my groove with the two of them. And DD is excited for her friend to come over
It's different than just having your own though for sure!
Have you talked to your mum or niece's parents about her routine (or lack of)? My neighbour was pretty laidback about the whole thing too, and her daughter sleeps and eats whenever, so I just did what works for me and fits in with DD tbh. If your niece doesn't nap every day can you lie her down for a while for quiet time? I don't know if she is used to that, but I definitely think kids respond differently to their parents than to other carers, so she might go for it with you even if she doesn't at home. Wrt when she eats - does she come to you with food or are you in charge of her meals? If it's on you then I would give her a morning snack, lunch, afternoon snack (and dinner if she's still with you) and that's when the food is available. She doesn't need to be picking at things all day. If she comes with food I would still try to structure it a bit but I would probably be less bothered as it's up to her parents what they feed her and how they want it portioned.
If she's not potty trained then I would keep using diapers until you're advised otherwise! It's not your responsibility to implement that. If your niece tells you she needs to go then absolutely take her, or if her parents want her in underwear then I would respect that but otherwise don't worry about it.
Is this going to be a permanent thing or are you just getting by until your mum is better?