If you were invited to a birthday party for twins (twin adults) but you only knew one of them, do you feel you should bring gifts for both of them?

I am invited to a birthday party for twins, brother and sister, who are turning 21. I am friendly with the woman, who is the niece of my good friend. I have never met the brother, or if I did, perhaps at another event, I don’t remember him.

I was planning to buy gifts for each of them, brother and sister.

Another of my friends who is invited to the same party told me she is only buying a gift for the sister, as she does not know the brother, and she feels the brother will have his friends there giving him gifts. She said she does not feel obligated to bring a gift for him, only for the sister.

I already asked the party hostess, my friend, what kind of things the brother likes.

Can you help me decide which is appropriate? To bring a gift for both brother and sister, even though I only know sister, or to only bring a gift for sister. Help!