I'm very sorry to hear about the death of your SIL's ex. It's He was quite young and it is never easy to hear about such things, but more so around the holidays.

Hopefully she will come to realize that life is precious and it does not stand still for anyone. Maybe she'll learn to appreciate what she has and begin to look within for her happiness.

Take care.

Sit quietly, the answers will reveal themselves when you least expect them to.
The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.