Pedo and incest aren't identicle but you can't call them completely unrelated. After all, families are things formed on the basis of children, and people spend more time with siblings and parents as children than adults.

Some heterosexuals ARE attracted to brothers and sisters. Those who are often suppress it or lose it in time or something, other times not. You can't know how your future self will react to things, and can't even estimate it since it's something you've never encountered. Those sure of what they'll 'never' feel are likely not going to be honest with themself if they are.

As for the mixing of roles, are they really that exclusive? Adult friend, fatherly figure... looking at it in their essense, friends are fun, fatherly figures are responsible for someone and try to help them. They don't seem incompatible. It's more a situation of what priorities you have in interacting with someone, and how certain aspects of interaction affect others. If romantic relations can lessen your ability to help someone then it would be bad. If they would enhance it there should be no question.

You really have to ask... if you have the capability to offer beneficial 'fatherly' traits toward someone, would it be selfish to just maintain being an 'adult friend' for your own fun? If so, there should be no uptight moral does and don'ts, it's quite fake responsibility.