Emergency boy help

Okay so one of my friends has two siblings a little sister and an older brother. I'm 15 and so is my friend. Anyways I'm an only child and so siblings are something kinda different for me I like her brother and I have a crush on him. He's 18 though. We started kinda talking about his sisters and stuff and then convo went on and basically he ended up asking if we could be friends with benefits and my heart started beating out of my chest I'm a virgin but I feel ready for sex because I've done similar stuff...what do I do?