There's no such thing as a sister not being a bridesmaid. Bridesmaids can be held by relatives, girlfriends or friends, from the responsibility, as long as they can help the bride, they can become bridesmaids.
Sister can be her own maid of honor.
First of all, my sister is her own family, and she will be very careful about her own affairs. It is also very reassuring to have valuables such as jewelry gifts managed by her sister. Secondly, it is very meaningful to have my sister with me on the most important day of my life. So there's no basis for saying your own sister can't be a bridesmaid.
Although in the eyes of modern young people, there are not too many restrictions on the marriage of bridesmaids, as long as you can perform the duties of bridesmaids in the process of preparing for marriage, as well as the wedding day, to assist the bride, then you can become the candidate of bridesmaids. However, due to the different marriage customs in many parts of China, the specific situation can also refer to the opinions of the elders.