I'm a girl and don't have that sort of trouble, (
girls) but... since you're a guy, and apparently an intimidating guy that girls are scared of, here's my guy advice from experience:
My ex-boyfriend took my little relative (4 year old girl) out shopping for her birthday, and told her he'd buy anything she wanted. She was in disbelief for a while, but when she realized he was serious, well
she needless to say, wholeheartedly obliged. They went to a children's "jewelry" store first (which is inexpensive), and then to a toy store or two, and in between bought a huge lollipop. After shopping, they took photos at a photo booth together. It was really sweet and cute. They had the biggest smiles.
When they got back and she was trying on her jewelry, he even let her put some on him, and laugh at him.
kind of going along with that, a word of caution...
don't make fun of *them.* If you're going to make fun of anyone, it better be of yourself. You're there to entertain THEM, not to get a kick out of them, and certainly not at their expense. For example, when the little one runs into the corner with her little doll, don't laugh. Girls are sensitive. It hurts.
oh, he also bought her (she didn't choose it) but he bought her a remote controlled car. Now, this was interesting because little girls don't usually care for or play with cars or other "boy" stuff. Well, it was a really fun car that can basically go upside down/crazy, and he nicely showed her how to "navigate." She loved it. They bonded, lol, and the car is forever associated with him. (The "jewelry" gets lost over time)
(or you can get a toy motorcycle, LoL... but I would still recommend a car)
Sooo, with all the presents, she still asks about him, lol. It wasn't all
that expensive. It's an investment
If you do this, you may want to talk to your brother before because it may cause them to like you more than their parents, or the parents may think you're "spoiling" them... (but so what)
Also, if you want to let the parents have some control over the presents, keep the receipts and give them to the parents. If they want to return something, they can, but they will also get the blame. LoL.
so, like others suggested, bribery works.