Do you think this is a fair way for my mom to make me spend my summer?

So, I'm a fourteen year old girl (I will be a freshman in the fall) and I am staying home all day every day to watch my younger brother. My parents divorced at the beginning of this year and my mom got a full time job. Now, she's out of the house from 7 'til 6 and it's my responsibility to watch my 11 year old brother. Honestly, at 11 years old, I'd figure he's okay to stay home alone for a few hours while I go out with my friends or something. However, I'm never allowed to go out. This includes when my brother goes out with his friends. My mom says I need to stay home in case something happens and he has to come home. She wont let me join my school tennis team, which I had been training for all year, in August because practice is four hours a day every weekday for two weeks before school. I think this is unreasonable because I have rides etc but she thinks my brother is incapable of staying home alone at this age (she would leave me home alone for hours at a time when I was younger than him, and my brother is not at all destructive or anything that should cause her to worry). Even leaving him alone for a half an hour stresses her out. Is there anything I can do to convince her to let me go out with my friends and not spend my entire summer locked up in the house? It's only been a week and I'm sooo bored....