Open marriages are fine if all parties are honest about the intentions and comfortable with the arrangement. You, however, clearly don't feel comfortable. So you need to be honest with yourself and say what you really want. If you want the actual full relationship, she needs to demonstrate that she is getting the divorce. Her children are grown, so they aren't together for their sake. If they have two places and are living apart, then I imagine they are not in dire financial circumstances. What is actually keeping them together? If she wants to be with you, it's time she shows it. If she won't, then you need to ask yourself why are you committing to someone who won't commit to you?

As for why they have things this why, here's my thought. He has romantic feelings for her, hence the jealousy. She cares about him, but doesn't have feelings of romance towards him. By her staying married but also having a boyfriend, she can get the romance lacking in her marriage without hurting the husband and ending the marriage. He meanwhile, gets to protect his marriage that he doesn't want to end, while remaining in her life. If they were together, they would have to face the issues between them which they don't want to do because it might mean ending things. This way they are, in essence, having their cake and eating it too.