Throughout history, people have settled for less than true love. I'm not recommending it, since DH and I are pretty much inseparable. But I'm not taking a position as to whether it's good or bad. Your cousin and his wife have to live their own lives and make their own decisions. If that ends in them deciding to forgo romantic love in favor of convenience and stability, that's their decision. A lot of people have done a lot worse than that. If they change their minds, your familial obligation is to be supportive and loving. If they don't, your familial obligation is to be supportive and loving.
You can't fix anyone else. You can't do anything about anyone but you. It sucks when you watch someone in a bad situation, but trying to interfere will just make it worse.