Should I have sex with my sister?

I have a friend that I need to owe money to because i borrowed some. For the past several weeks, he has been yelling at me to cough up the money i owe him. I've been looking for ways to get a hold of twenty dollars but it just doesn't work. He has even been threatening me that he will quit being friends with me if i don't give it to him within the next week, So i need the get the cash really fast. Now, my sister heard about this problem and she told me that if i were to have sex with her, she will give me fourty dollars, so i can keep half of it for me. And she ment real sex, not just a quicky. I told her i'll think about it and let her now soon. I'm in a relationship so that is what's bothering me. My sister is adopted, and she has only been my sister for about 7 months now. So i don't think it's creepy at all if i were to do this. And she is VERY pretty, and most of my friends think so too. And my sister is 16, and I'm 18 so it's completely normal. Our parents leave the house very often so I could just have sex with her on our couch after she comes back from school. I will plan on bringing a condom if i decide to. Should i do this? Could this be bad for us in any way in the future? I haven't had sex before but I know how I guess. And how exactly should i do it with her if i were to?