Sounds like this sister may not be so happy in her marriage?
My advice is that a couple, once married, should never go to bed mad at each other. May lovely sweet wife and I have been married for 27 years and attribute it to following this rule. We have never gone to bed mad but we stayed awake for 6 weeks once.....
Seriously, we have been married for 27 years and I was 19 at the time of our wedding. We give great advice on marriage, but we can't remember what it was like to be single anymore, so that we leave up to others......
Don't discount the sage advice of old married couples!