The Simpsons if they weren’t terrifying yellow monsters

Nah, if both parents carry a recessive gene then it’s very possible to have a kid with different features. But this is rudimentary biology from 5 years ago talking, someone here will be able to inform you more than I can

My hair is dark brown and my older sister is dark blonde our parents were both dark haired so it's very possible for one sibling to be light haired and one dark without the 'milkman' factor.

But what if your sister did come from the milkman? How would you tell?

If anyone in my family has suspect paternity its me, my father and myself are polar opposites and have never gotten along. But my sister and myself are both his kids no questions we share some distinctive family traits such as a birthmark on our right ear (its odd but all three of us have it, even my kid does)