My D rarely slept for longer than an hour, 3 times in 24, right from birth. By the time she was 12 months old I was exhausted, mentally and physically, and living on my own.
One day of no sleep, I finally snapped. I dismantled her cot and bed, removing them from her room and left her with the mattresses and cushions on the carpeted floor, duvets and blankets plus soft toys, books and a low light bulb made up her new bedroom. Her bathtime routine was moved to 8pm then story, cuddles and into her bedroom by 9.30pm and a stair gate fixed to the door. She very quickly settled into a 3 hour sleep routine anytime between 9.30 and 6.30, playing quietly in her bedroom when awake.
She asked for a bed at age 7, up to that time, she loved her room as it was. Drastic solution but it worked for us, and yes I had health visitors checking on us, and all agreeing that it was a practical, safe solution.
She is now a happy mother and wife, with 4 lovely GC who all sleep well at night!
OP, I'm sureally your DD and SiL will find a solution, maybe offer to settle the children while they take an hour or so to go for a drink or quick meal. My daughter and SiL do that and it's become a catchphrase that even the GC use. "Are you taking advantage and going out now Nanny's here?" 7yr old granddaughter asked 6 weeks ago. They laughed and yes, they "took advantage" and went for a quick meal and drink.