Well maybe it would start with him complimenting her a lot. Then asking her what he could do for her. Gwen would be skeptical at the sincerity at first, then flattered, then insist she's fine. But then she'd jokingly demand a foot rub and be shocked and flustered when he'd hop right to it. He'd be surprisingly good and she'd be addicted in no time, with Ben only happy to make his heroine feel good. Max would just be happy they get along so well and not notice them getting closer and closer. In the middle of one of Gwen's pampering sessions Ben would mention how cool as Fourarms she was today and Gwen would get the idea to do something nice for Ben in return. She'd transform into Fourarms and just generally use her body like a carnival ride for him: throwing him in the air, showing him tricks and feats of strength, letting him feel her muscles. Ben would really enjoy it. Maybe a little too much and Gwen would notice. She'd start to pay closer attention to Ben. Try out moaning a little louder during massages, giving one back perhaps. At which point Ben's crush on his amazing cousin would become obvious. And soon would grow to all kinds of fun times with Gwen as she reciprocates the feelings of her number 1 fan. Not even with her taking charge, but with Ben putting her in charge because all he wants to do is worship her. Both in the form of his amazing cousin and in the forms of the hot alien ladies she sees her do heroics in. And Gwen is happy to oblige.