I think when a girl reaches 13 she should be allowed to wear whatever type of panties she wants, thongs included. I started wearing thongs when I was 13. I didn't always wear them but I wore thongs whenever I wanted, and I think your sister should be able to have the same privilege. Just keep buying them for your sister if your mom won't, and in the meantime start trying to convince your mom. I think a little persuading on your part and if your sister keeps bringing it up, it won't be long before your mother will realize that she out of date. Of course they're sexier, but goshhh she is 16! I think your mom is thinking how it was when she was your sister's age. You should try to make her realize that times have changed considerably since then. Girls talk a lot and most wear them, so you don't want your sister having to lie to her friends that she wears them when she really doesn't.