My advice to you is...you are her sister and you love her very much, it shows because you are so concerned about her. She is in love, and they want to be together, although she is only 22yo it is possible to be in love at that age. Your intentions and opinions clearly come from your heart - but you need to leave it, and let her and him get on with it and if it doesn't work out then so be it, but if it does fail it will fail in it's own time and not by you telling her to finish it. Let her make her own mistakes, as much as it hurts you. You are her sister, you just be there for her as she needs you, and maybe to even pick up the pieces if this does fail.
Maybe their marriage will work? Who are you to say it won't (no offense intended)?