i don't think a relationship can be there only because someone told you that "wants to be with you". there should be a lot of other preconditions which should be there to make a relationship real:
1) are you willing to share what you are thinking to each other frankly? 2) are you willing to share your finance conditions to each other frankly? 3) do you think it is very important and very easy for you to make each other happy?
i think if the preconditions haven't been there, you might not need to tell "yes" to her, maybe you can only tell her that you haven't prepared all these.
Ok so lots of people your age say no.Im much much older and I also say no.Mostly because it never feels right with the entire family if two sisters are attracted to the same man.It happened in my family.When my marraige broke up,my husband and my sister showed an interest in each other.It felt totally shabby and it was.Eventually she dropped if but Im not sure he would have.Nowadays it seems like some girls plan it,which is somehow bad news for you,altho I dont know where the plan leads.Figure if you are young you can pretty much date anybody you want.But dating is not about hurting people's feelings,and it seems to me as tho that is what these girls are somehow trying to do to you.Stay clear of it and uncomplicate your life.This is what I think.