When I had two cats, They did not think of each other as siblings. One of them (skidder) would dominate the other and bully her (Coletta). Skidder was a nice cat, but he wanted all of the attention. Coletta knew they were not siblings, in fact I'm sure she realized that she had been separated from her own brother.
My point is that cats can understand who is family and who is not. Think of it like this: If you were an adopted child, you would look different from your adoptive family, at some point you would realize you aren't truly related.
Cats are similar, even if they believe it for a while, they will realize they aren't related.
However, I believe raising two kittens is a great idea, just be sure to treat them equally, even if one acts more hostile.