Me: Sad, but I will survive
No matter what, we are always our children's parents first. I'm mom to five children. Three of them are adults. I've essentially been a single parent to them for seven years now, and as a BW, and now BGF, I feel strongly that they can learn from what I've gone through.
My older three are ages 24, 21, and almost 18. When it comes to this generation, most have a warped view of what's right in a relationship. I discovered this when I entered into a LTR with a much younger man. He made some serious mistakes in the beginning, but looking at his friends, and talking with my adult children, his actions and views were common amongst this younger generation. Age isn't an excuse, but when they aren't being taught different by us parents, and their friends all treat commitments as something that's only when you're by your partners side, how will they learn any different.
So with knowing this, I think without a doubt, you should talk with them. I'd talk with your W first, and maybe take your daughter and SIL out to dinner as a team. At some point bring up how much you want them to be happy, and succeed. Maybe give them the book "Not Just Friends" as a gift. Tell them how you've made mistakes (without intimate details), show emotion, let them know that your spouse is number 1 ALWAYS. And talk with them about what it means to be friends of the marriage.
I know kids will roll their eyes, maybe even think we are crazy... but somewhere in their head, they will hear you. Who knows, maybe they will love you even more for it.
Three things may come of it:
You may end up giving them enough insight to prevent something harmful.
You did your job as a parent. You will never have to sit back and knock yourself for not discussing it.
And you opened up the door, in case they ever enter down this horrid road, to be a person they can confide and come to for help.
Thank you for being a parent who wants to stomp this before it continues.
True Love: What I have for my beautiful children.