I would not recommend just putting them together. Even with spayed or neutered rabbits, this does lead to some pretty nasty fights and sometimes large vet bills and even death. While rabbits can be social and some do enjoy the company of another rabbit, they need to be introduced slowly. Bonded pairs also need a larger cage than a single rabbit (about twice a big if not bigger). With a pair, especially a new one that is in the process of bonding, you need to be 2 cages and be prepared that they won't bond.
The best pairing is a neutered male an spayed female. If both are the same sex, they do need to be neutered as well. You also can't have a recently (within the last 2 months) with an intact female as he could breed with her and produce babies since males can still be fertile for a little while after being neutered.
If your sister wants a rabbit, then you need another cage. If you want to bond them eventually, that takes time and you will need a bigger cage or area.
