Loads of people have doppelgangers. I went though secondary with numerous people thinking a girl was my sister - she 100% wasn't. There were constant conversations along the lines of;
"I saw your sister in the park last night."
"No you didn't, you saw X, that's not my sister."
"Yes she is!" 
We were like twins, - same build, hair texture, length and colour, same cheekbones and face-shape, green eyes. But she was categorically NOT my sister - although she was from a Welsh family, she was born in Australia!
The only people this concerns is the mother, the child and his dad, who may (or may not) be your brother.
If your brother is the biological father, and she has decided not to tell him, that's her prerogative. The boy may well have another man in his life who he looks to as a Dad - he may even think he's his Dad.
Leave it, - you have no idea what is going on in this family, you saw a child in a car park.