If you are in school (or a church), is there a chorus/opportunity to take lessons?
If not, is there a group in your community that gets together to sing? (try looking on line; I know that there is a group near where I live called 'songweavers' who get together to do just that)----and certainly sing, don't worry about sis, she would hardly be your most objective critic...
You play an instrument...another informal venue, if there are some near you, are "open mics" where those who play and sing get together, take turns, or play together...
Friends who also play an instrument/sing could be gathered together in someone's place weekly & you could all practice in a relaxed, non-judgmental atmosphere...

You should get singing lessons to build your confidence, don't worry about your sister she'll be jealous when she hears how good you get!! Stay positive and use your music talents to help you make the most out of singing, whether it takes you somewhere or is just for fun!

Singing (even when done badly, which describes me) is good for the soul.