Girls

Two same gender kids can share a room and a bed. If this is a permanent thing thought, you will run into problems because of the age difference, one is an adult the other is pre-teen, so different things going on making it hard to share a room. The best would be to have separate rooms.

Are they siblings? Is this permenant? If it's like a sleepover or something, I wouldn't see a problem, but if this isn't a temporary thing, I would say no. Both girls need their own space and privacy. A little more info would be useful :)

No, not the same bed, maybe temporary the same room like if the 10yr.old is over on weekends.You are asking for a child to share the same bed as an adult. How about bunk beds,futon, an air mattress, the couch.
This is what I personally did . I lived in a 1 bedroom apt. with my young daughter (I saved up for a yr. to buy a house) I gave her the bedroom and I bought a couch with a pull out bed in the livingroom she was 2 at the time and I was 20.
I hope you find an arrangement that works for you.

not OK for about a million reasons.

I shared a bed with my sister from the time I was 3 until we were able to have separate rooms when I was 12. She is 9 years older than me and it wasn't a problem. Yes I did get on her nerves when she had friends over however, there really wasn't another option. We lived in a two bedroom house with an added on family room that my brother slept in.

Good luck