Hi. I was living with a family. There I had a foster brother, who did not live at home. I am 17, and there fore able to enter into any relationship legally. I think I have developed feelings for him, that are not meant for a "brother". Is this wrong?

No, you aren't related, and weren't brought up together.
But where is this? Age 17 in many places is still a minor child and a man can be charged with statutory rape if there is sex between you. I'm not sure what you mean by 'relationship' but please wait until you are 18 for intimacy.

I am from the UK, When I say relationship, I just mean "BF GF" type thing. Plus, I ain't ready for a sexual relationship anyway.

As long as the foster parents you are living with do not care. At 17 while "free" you also are under their care.

I'm not actually living with them anymore. I moved out in May xx

I think u should wait and if u still have feelings for him then give it a try but hopefully you guys r still close after