Hello there Bets - it is an interesting situation you have there. Perhaps in families where there are Asperger members, it tends to rub off on other people in the house. My younger brother has coped very well with the fact that I have a disability, but he is very mature for his age, and I image most siblings (particularly younger ones) find it all very confusing, even wondering whether they have it themselves.
If you are not sure, you can ask yourself whether you think you had any aspie traits when you were very young (e.g. speech delay). If not, and the traits are only recent, the chances are it has just 'rubbed off' on you over the years. Either way, its nothing to worry about, honest 