Could she be adopted?

Okay so I've recently realised that my two aunties who are sisters. Are both 50 this year. One was born in june and the other was born in November.

I always wondered why the Aunty born in June was the odd one out. She's got dark almost black hair while my other Aunty and my mum have blonde light hair and the photos of my two aunties and my mum has children, my Aunty looked different (as in not as similar as my other Aunty and mum looked)

Does this mean she was probably adopted?

I don't feel comfortable asking my mum or my grandma, but how else would they be born 5 months apart?

Probably sound dumb and it's probably a dumb/obvious question but yeah

Btw I'm 17