Hi there. You can't get pregnant from anal sex. You'd have had to have vaginal intercourse for pregnancy to occur. Her parents don't need to be notified for you to use a condom and her to use spermacidal lube. :>) Get the condoms and speramacide and prevent pregnancy as best you can. She is not pregnant from anal sex, that's not a worry. But to protect her if she will be in danger if her family finds out about this, use other forms of birth control or abstain from having sex. She also can tell a school counselor if her parents are abusing her. I'm sorry if that is happening. Make sure she is safe and don't knowingly do things that put her in more peril with her parents.
So, recap. She is not pregnant from anal sex. And symptoms of pregnancy do not show up immediately. They normally show up at weeks 6 or 7 into pregnancy. Girls of 15 are often just late. If she begins to have vaginal discharge that has an odor then she should get checked for a vaginal bacterial infection. If you've had vaginal sex that you are not telling us here or in the future, she can take a simple home pregnancy test bought at any drug store, grocery store or super store like Wal Mart or Target at 2.5 weeks after the sex and these are accurate. You do not need parental consent to buy a pregnancy test (and they are not expensive).
good luck and buy some condoms.
Quite a bit of sex may have included some vaginal penetration, so running a home pregnancy test would be a good move when her next period is due. Yes, you need to focus on good contraception from here on out. Do you need help with that? Sorry about the abusive family.