Being a godparent doesn't require you to do that. Being a godparent should just mean that the parents feel closest to you and they knew you'd love their daughter too.

My godparents never gave me money, that's not what they are there for. The most important job of a godparent is most likely moral support more than anything. Sure you can give your godchild gifts, IF that's what you want.

Like my godparents kept getting me gifts for christmas(usually only parents in my family buy gifts for kids and the grandparents too). They weren't obligated to do that, they just like to do it. I once told them that it felt wrong for me to get more gifts than the other kids in the family and they said they just wanted to do it to make me happy(then again they never gave me anything costly or anything, that would have just made em feel bad, they always gave me handmade stuff, usually some of my favorite foods like meat pie because I love my godmothers meat pie so fucking much). I was happy they gave me gifts of course, but the most important thing was that I knew if I needed their help they would be glad to help me and just knowing that is more than enough for me.

What you should do in your situation is just do whatever you want. If you want to give her a gift, go for it, if you don't want to, that's fine to, just let her know that you love her. And honestly, 7.5k is a ridiculous amount of money to give to someone unless you have plenty to spare and that person has done something that made her REALLY deserve it or she's in dire need for help and you want to help her. Getting to 18 years old is not an accomplishment worth that and I doubt she's in a situation where she needs that money. So tell her mother that it's nice she decided on her own to give 7.5k to your brother, but that you don't feel comfortable giving her that much money because you don't have that much money to spare like this.