If you don't have money to buy a mobile phone and you are embarrassed to tell your girlfriend directly, then I suggest you customize a special birthday gift for your girlfriend's sister to "make up for it".
Here are some special birthday gifts for your sister, I hope it will be helpful to you:
1. Personalized color-changing cup: You can print your sister's photo and blessing words on the cup. The cup is usually black. Once hot water is poured into the cup, the cup turns white, so the printed photos and words will appear, which is very magical. It may make your girlfriend's sister very moved, and your girlfriend may be jealous, because you didn't give her such a good birthday gift, haha!
2. Sterling silver name necklace: You can customize it with your sister's name or English letters. It is a birthday gift designed specifically for her. She will definitely like it. It is also made of sterling silver, and it looks good and atmospheric when worn.
3. Blessing chocolate: Customize the blessing you want to give to your sister on the chocolate, for example: Sister, happy birthday. Chocolate is a favorite of many girls. It is very sweet. Giving chocolate to your sister is equivalent to telling your girlfriend that you love your sister as much as she does! (But don’t forget to buy a box for your girlfriend as well.)