Kittens typically aren’t given away before they are a couple months old, so they most probably met each other.

Either way: I’ve seen this in action. We had a cat that had kittens, and we were unable to give away one of them. When the kitten started growing up the mother started harassing it, eventually to the point of chasing it off. Luckily we found out that it moved in with some people a couple streets over that were very happy to have it. The point is: Cats aren’t pack animals, and typically don’t like sharing their territory with other cats, even if they are related.