If someone were to gain an unexpected family member, that someone may want some advice on family.
Except there might be some snags. The two could not know about each other until adulthood, which does happen. That's manageable, though.
Someone could be trying to kill them both, which is less common but is still manageable. Plus, it provides some great bonding opportunities.
Then, of course, one of them could be one of the most powerful wizards on the planet and the other could be a vampire who feeds on emotional intimacy.
That one's a bit more difficult.
Still manageable, though.