I agree with Samuel's mum.
Keep trying with the 2 -3 hourly feeds. Not only is dehydration a potential problem, but jaundice can occur. Babies need milk to be able to process the bilirubin from their bodies. If they don't get enough, the bilirubin builds up causing jaundice. A jaundiced baby is VERY sleepy and then the vicious cycle occurs. Too sleepy, doesn't feed well, more build-up, too sleepy etc.
I would try pumping. It's going to stimulate milk production. With DS1 I had a lactation consultant tell me to feed baby first, then pump afterwards. Kinda tricks the body into thinking that you are still feeding the LO.
Syringe feed or gently let baby "sip" from a cup. This will help prevent nipple/flow confusion.
Some people's milk takes a bit longer to come in. Stress won't help. But that's so hard to do when you are worried about your baby, tired from birth etc etc. Tell Mum to make sure that she is drinking lots, eating well and getting as much rest as is reasonably possible.
You are a fantastic SIL/sister to be looking for hep for them. Keep up the support. We all know how important that is.