Most Helpful Girl
Most Helpful Guy
I think you need to careful about it. If they are a couple that are like swingers or have an open marriage and is absolutely clear about it with you then maybe you have the permission and the go ahead to get that close and develop some kind of relationship with her. Otherwise, like everybody else here pretty much already said, time to move on and look elsewhere, although you could still remain as regular friends, maybe close friends but nothing more.
And I don't know if her husband is really absolutely ok with it. If he was really jealous and impatient, or perhaps even abusive then you set her wife and potentially yourself up for physical harm and danger.1