It is the younger generation's responsibility to congratulate the elders at home on their birthdays, not to mention that your sister invited you to the party. Bringing a birthday gift to your aunt is also a courtesy. Here are some suggestions:
First, give some home accessories. People of your aunt's age must care about the decoration of their home. You can give her a crystal music box. Beautiful crystals and good music will make her like it very much. You can also give her a large plush toy. Although it is not suitable for your aunt's age, it will add some family atmosphere. There are also some other good gifts.
Second, give some household gifts, such as a set of blue and white porcelain tableware, or a set of new bedding, which will be good gifts.
Third, giving some flowers is also good. Of course, you can apply to buy a big cake for your aunt yourself and write a congratulatory message. It will definitely make your aunt very happy. You can also give chocolates, which can be customized with birthday wishes.
These are some of my suggestions. I hope they can help you.