I wonder why youre mom is so against people on cons, even though she seems to have never attended one. Have you ever asked her what makes her think so negatively about congoers and cons? She doesn't seem informed about it at all, so I really asked myself while reading that where her opposition against cosplayers comes from.
What the others said about sewing practice sounds really good and I'm glad to hear you decided to stay. Don't let your mom destroy your dream of cosplaying. The day you become independent and can finally start cosplaying, you'll surely prove her wrong