My husband and I are in a similar situation. We lost our first son @ 26w in May in a car accident. He was born via emergency c-section, and lived for 12 mins... Although neither of us go to meet him alive as we were both in surgery...
We're are currently WTT until May of next year - per doctors orders... We are sure we want more kids, and that our other children will never replace Grayson...
I read "Empty Cradle, Broken Heart" - there were parts that were very helpful... but it's difficult when your loss is trauma related, as most help books are more about miscarriages & genetric losses. Every loss is unique, and each person's reaction will very, but I still feel there's a difference in the traumatic loss. There is a section towards the end on subsequent children & pregnancies that I found very helpful.
Something important I took away from the book was - your friends and family will only talk about your loss as often as you do... Your SIL & DH may want to talk about their loss, but feel they don't want to bring everyone's mood down... but if they don't talk, then you don't talk, and then they may feel neglected... It's really a tough situation...
Good luck to them, and may Karma ensure the remainder of the journey is a little easier...