Birthday gifts for your boyfriend's sister, no matter what you give her, she will like it very much, after all, it is a gift from her future sister-in-law, so she will cherish it especially.
It's winter and Christmas is coming soon, you can give her a scarf, scarves represent warmth and care for family affection, and a cute scarf for girls in winter can add a lot of sunshine to her, and you can also give her a glowing teddy bear, which has always been a favorite of little girls, and glowing is even more lovely, especially at night. It is particularly dreamy. You can also give her a name necklace, every girl should have a unique necklace of her own, it is a very memorable gift.
If possible, take her out to play on her birthday, which will be more meaningful than a birthday gift.