AIO for being a bit upset that my brother’s girlfriend keeps buying me stuff

Ok so I (19F) have a little brother (15M). He introduced our family to Carina (14/15?F) a couple months ago. They’re sophomores in highschool and I don’t think they’re even officiating dating yet but they’ve said they like each other and they act like a couple. They’re at each others houses every day and are inseparable. They call every moment they’re not together and I’ve woken up to her just walking around our house lmao. I’m not close with my brother but recently we’ve been on better terms, we’re just opposite in personality and fought as kids. He’s also always been nonchalant around family and I’ve always been a talker, so I think we used to just get on each others nerves. But like I said, it’s better now as we’ve gotten older and learned to adjust to each other more. Anyways Carina is really nice and sweet. I like her a lot, she’s nice to my brother and smart. I don’t really judge other people’s partners or interfere in their alone time bc it’s not my business but I say hi to her and make a little small talk before going back to my room. Here’s the issue?? though, yall let me know but basically my brother texted he was in Target so I asked him to get me some strawberry ice breakers and that I’d pay him back. But when they got back he told me that Carina bought it and wouldn’t let me pay her back. I tried to light heartedly say I needed to pay her back but they both kept declining. So a week later I went back to target and got her some chapstick and nerd clusters. A few weeks after that my brother asked me what I wanted for Christmas, which was weird bc he never gave Christmas gifts to anyone in the family. He then told me that it was Carina who asked and she wanted to get me a Christmas gift, I said no she didn’t need to. But he kept asking so I told him some cheap items and to send me on text what she wants. But she refused to tell him what she wanted so he backed her up and they wouldn’t let me get her anything back. I obviously kept saying I need to get her something back and that she didn’t need to get me anything. Which this convo didn’t get brought back up and it’s not Christmas yet so idk if she’s serious. Then today they were at Starbucks and I asked my brother for a mocha frap and I would pay him back plus a little extra and he said yeah. But then he said Carina bought it for me and that she didn’t want to be paid back. This time I was even harsher through text that I would pay her back. That it was nice and tell her thank you but she needs to be paid back. She refused and my brother said I can pay him back. I relented because the drink was already bought and said I would pay him back with extra BUT THAT SHE NEEDS TO STOP BUYING ME STUFF. He just said “na.” And sent some cute pics of them to distract me. But then he said the drink actually wasn’t bought yet and I spammed not to buy it. But they stopped answering. I get its nice, I know she’s trying to like win the family but I already like her, I don’t need gifts. And she’s literally 14 or 15 so this isn’t her money 😭 And I didn’t ask her to get me free stuff, I just asked my brother to pick up stuff I would pay for. I feel like I’m too old to be receiving freebies from my little brothers girlfriend. And the fact that they won’t let me buy stuff for her or pay her back kind of makes me feel like they’re not listening to me. I’m not mad obviously, I didn’t yell at them or anything, but I just am a bit peeved inside. It’s nice but I’m just gonna have to inevitably return the favor. Idk guys how should I approach this, am I overreacting. I just don’t want her to spend money on me basically and like I didn’t ask her and why is my brother blocking me from returning the money