Godseeker,
Why are you here at Marriage Builders?
Your post isn't entirely clear about your purpose for being here. Are you looking for validation that you are right that your SIL's marriage is "heading for disaster"? Are you looking for encouragement to intervene and make your SIL's life "right with God"?
I won't give you either.
Interventions "for his/her own good" rarely ever work. Also, *you* are not a part of your SIL's soon-to-be marriage. You are an outsider even if you are family so please be careful in making judgements. It is her life and she will make her own decisions whether or not you like them.
If however you are looking for ways to help and support your SIL in her effort to build a loving marriage I would suggest that you take her out for lunch, support her, congratulate her, and give her an engagement gift (
The Four Gifts of Love: Preparing for Marriage That Will Last a Lifetime (book) and/or
Fall in Love, Stay in Love (book)).