For nearly a year...

For nearly a year I have liked my friends younger brother. He is two years younger than me and I have tried desperately to get over this, but nothing has worked. I don't want to ruin my friendship with his sister, and they don't always get on well. I know that it could come down to choosing between them and that would be really difficult... I was talking to his mum some time ago and he said that if we did go out it wouldn't be that bad, but I don't know what other people will think. Some people have told me just to ask him out, but I don't know if he feels the same. Part of me thinks I should just take the plunge, while another part just thinks that I'm not pretty or good enough for him. I know they say that if it lasts longer than 4 months it is not just a crush, but I don't know how to take the next step, or even what it is. Should I chase my feelings or just try even harder to make them go away?