Hi, K.,
It's wonderful of you to look for resources for your relatives.
I think that almost everyone who has every been married agrees that marriage takes work. The question is "What type of work?"
As a graduate psychology student and wife, I've read many books on relationships, esp. marriage. If your sister or brother-in-law are at all open to reading to help them, I recommend that they read the following books:
1. The Seven Principles of Making Marriage Work by John Gottman. This is my favorite book on the topic. My sister, who is a clinical psychologist in San Diego, gave me this book when I got married.
2. The Five Love Languages by Gary Chapman.
3. When Your Sex Drives Don't Match by Sandra Pertot. (I don't know whether this topic applies to your sister and brother-in-law, but it's a useful book for many distressed couples.)
4. I Love You, but I'm Not in Love With You by Andrew Marshall. (I don't know whether this topic applies to your sister and brother-in-law, but it's a useful book for many distressed couples.)
I would avoid books whose authors offer platitudes or quick fixes, generalize, e.g. "Men are (fill in the blank)" or "Men are simple creatures," basically repeat what they said in the first ten pages over and over for 200 pages, or do not have PhDs or solid research backgrounds in PSYCHOLOGY. A PhD in another field, e.g. mathematics or finance (the majors of my previous degrees) does NOT make one an expert in psychology! That's why I'm going back to school, yet again, to get an advanced degree in psychology. I don't want to give people garbage in exchange for their hard-earned money. However, sadly, some people pass themselves off as experts in order to quickly make big bucks. They may sell lots of books, tapes, etc., but I don't think that they really help distressed couples and further the general public's understanding of what makes people tick or even just help improve relationships with a significant other.
If your sister and brother-in-law are more "workshop/seminar" type people, then they might benefit from a workshop such as John Gottman's workshops, which are held in Washington state and California. You can get details on www.gottman.com.
I wish that I knew of what I would consider a good marriage therapist in the area. I would go to him or her!
Good luck to your sister and your brother-in-law.
Lynne E