<human>: the two of us are 18, as is her sister. we're doing absolutely great and are enjoying a few more weeks of summer together before she moves out of town and starts college a few states away. obviously we'd like to enjoy that time without any commotion.

her sister is basically putting the rest of our summer in danger. she insists on continuing a relationship with a teacher that started halfway through last school year. their parents know she talks to him, she's been caught sneaking off to see him, and they told her that had to stop right away. I'm afraid the next time she's caught they'll just be pulled out of town early and that'll be it. they're twins and that's how their mom punishes them - yes, even at 18 - in pairs.

I'm finding it hard to control my anger about the other relationship. her mom is already suspicious and watching like a dog and she's inviting him over when nobody's home like there's no risk at all. and as much as my girlfriend and I try to stay away, their mom expects my girlfriend to tell if her sister tries something funny, but obviously sisters don't tell on each other.

so basically we (and I, since it's easier for me to be mad at the sister than her) are fighting against the two closest people to her, even though we're trying as hard as we possibly can to not sneak around or get in trouble. I sit here all day waiting for something to be found out and the hammer to come down. and there seems to be no budging the sister. help? :(

Summarize the given text in a few sentences.
<bot>: my girlfriend's sister is in a relationship that jeopardizes the time we have left together and there's no budging her about her decision to sneak around with him.