How can i help my granddaughter who has selective mutism?

my daughter is a single parent and has two little girls that are 4 and 6 and they have lived with me ever since the youngest was born so i have practically been raising them myself because my daughter is going to school full time and wants to live her life. i love them as much or more than if i had them myself but i don't know how to help my youngest granddaughter who has selective mutism.I sent them both to preschool since they were two but the youngest one has never spoke a word at school since she started three years ago.she didn't mind going to school when her sister was going but now that her sister has started kindergarten at public school she is fighting having to go to preschool. I would take her out of school but I'm afraid she will not be able to keep up in kindergarten when she starts next year and I don't know if that would be good for her selective mutism either. The main reason I keep sending her to preschool is that she is learning.I don't know how she learns since she doesn't participate but she is. she knows her colors most of her letters,numbers and shapes. she knows matching what's missing ,patterns and can memorize bible verses and even whole books but she only lets us at home know this.I mostly want to know if I'm harming her or helping her by keeping her in school. The doctors we've seen here don't seem to know very much about it except that she has this disorder but can't seem to offer any advise as to what to do about it and how we can help her. I'm afraid of how her future will be if we can't get her out of this.can anyone please let me know if I'm doing the right thing by making her stay in school. thank you