Should I tell on my sister??

My mom is very serious about asking to do certain things, like to go out somewhere or date someone or get a social media account. My 10 year old sister recently got a snapchat without my mom knowing and my brother knew and told me not to tell my mom, even though we both had to go through the process of asking for a social media account and have both gotten in trouble for getting one without our mom's permission before. My sister and I already aren't very close and I don't want to make things worse, but I don't think it's fair that I have to deal with my mom watching over my social media accounts while my sister has one without my mom knowing. She also doesn't understand how sharing or saying certain things through social media can bring harm to herself and/or others. I'd talk to my sister myself, but she doesn't listen to me at all. Should I tell my mom about my sister's snapchat account or keep it a secret?