My best friends brother

So I just found out today that my best friends brother likes me as more than a friend. We are quite close but it was only friendship (well too me it was only friendship) but today he told me that he likes me. I tried to let him down as i am noy ready for a relationship yet but i feel like it is awkard between us now. It has only been a few hours but I feel kind of guilty. I also don't know if i should tell his sister (my best friend) or not. Any advice on where to go from here?