I live in a house owned 50/50 by a brother and sister. The sister has moved out so it's just myself and her brother and she has asked me to pay rent just to her and her brother to receive nothing as he is still living in the house. Is this legal?
There are two possibilities here. One is that the brother and sister have done a deal where the brother gets to live in the house rent free (because he owns part of it - essentially he's paying rent to himself) and the sister gets your rent paid to her as income (for owning a house she doesn't live in). That's pretty normal and makes a lot of sense. The second possibility is that there is some kind of argument going on between the brother and sister.
It's easy to discover which. Go to the brother and say "your sister asked me to pay all the rent to her - is this OK with you?". If he says "yes" it's the first case. Relax and enjoy the new space in your house, and pay the rent to the sister. If he says "No, what the hell?" then it's the second case. Start looking for a new place immediately, and in the meantime pay the rent exactly as you were doing before.
You're in big trouble here. Do you have a signed lease? If so - act as agreed in the lease. If you don't have a signed lease - then you should move out ASAP or get one. Otherwise, you'll find yourself in a cross-fire between the arguing siblings.
Legal? Laws of men have nothing to do with it. Its the laws of nature. When family is in a feud - get as far as you can.
If you can talk to the brother and get him agree to the arrangement, you might get out of the situation unharmed.