As far as I can remember, the attraction/lust was all one-sided, all in Robbie’s head. As far as I can remember, Fran’s brother is straight and unaware that Robbie might not have been completely smitten by his sister. Just the usual horny Robbie drooling over an unavailable guy! ;–)
Robbie could go as Lady Godiva and kill two birds with one stone! He could finally fulfill his promise to pose for Fran to paint a nude portrait. As Lady Godiva, he could flirt with the brother – and maybe score! ;–)
Don might not be a church-goer, but he clearly believes that a traditional male-female marriage solves all problems. Don seems to think the major issue that Robbie had was the lack of a father, not that his mother died and left him an orphan forcing him to move across an ocean to live with a family he didn’t know – but Donny is the sole, obvious, and perfect choice to resolve that problem. All the other issues Robbie thinks he has will melt away if only Robbie also subscribes to that philosophy. In your wildest fantasies Donny-boy – only in your wettest dreams! ;–)