I don't know my half sisters birthday
Do you want to know it?
Not really, she is just another standard person in my life.
Everybody forgets some birthdays, but you should at least know hers.
I don't really care. She is a bitch anyways.
Even though she can be disagreeable, she is still a part of your extended family, so you should at least know her birthday, and make a token gesture toward her on that date.  You wouldn't want people you know to completely ignore your birthday, so it's good for you to treat your half-sister with the same respect.