I've gladly adopted the sisters my S.O. came with. I only had a half-sister growing up who wasn't really the nicest. He has three sisters, so I like to jump stores and stick to color themes for them.

Pink, Purple and Green (each of their favorite colors). Anyone who has a sale of small items in each of those colors, I buy one of each. Once I have a collection of the items (soaps, bubble bath, socks, jammies, plush toys, etc.) I buy big baskets from the 99 cent store. I line each one with a bathrobe of their color (Target) and fill it with goodies. Then I cover/wrap it with a large towel of their color (whoever has sales on towels) and tie the top with a hair scruncy of that color.

They get excited when Christmas comes, he always gave them gift cards before we met. As soon as we show up they start peeking for their baskets.