Need advice. Son and his wife can't-won't follow sleep training suggestions for 3 year old daughter. New baby coming in February

Nov 11, 2021
Our problem is watching our son and his wife -both in their early 30s unable/unwilling to follow through on advice from many sources to establish sleep regimen for 3-year-old daughter. Son has a very successful donut bakery and must be up at 4:30 3-4 days per week. Daughter-in -law has a full-time 9-4:30 work from home administrative position.
And then there's Nadeen -just turned 3. Lovely, smart, funny. BUT, her parents keep giving in to her---they are dragging every night especially the mother. Our concern is that since a baby brother will arrive in mid-February we fear that this change will cause extreme turmoil for all three, and any hope for change will seem hopeless.
So far we and the other grandparents (that are equally concerned) have been unable to reach my son and his wife. They adore Nadeen and, from the outside, look like a great little family--which they mostly are.
We plan to discuss this with our son and are unsure how to speak to them about this. Obviously, we have to highlight the probable negative consequences for Nadeen and also for all of them after the new baby arrives.