My first baby was like this, if she wasn’t asleep or feeding she was crying. If we picked her up she cried, if we put her down she cried. She cried in the car and hated her pram. She also fought sleep viciously, every nap was a battle and she didn’t self settle for months.
She had no medical issues as far as we know and she gradually just grew out of it. Now she is a perfectly normal, happy 6 year old. Still quite intense at times but smart, funny and very loving.
I’ve recently had a second baby who has been a dream in comparison, so much more settled, sleeps easily, self settles, loves the car and her pram. I’ve done absolutely nothing differently and have come to the conclusion that some little ones are just grumpy because they dislike being a baby.
It might be worth investigating silent reflux if the baby completely refuses to lie flat, but if he is sleeping 12-7am then this seems unlikely to be the case.
The not settling until 12am is quite normal in the early weeks, both of mine did this, until bedtime started to naturally move earlier after about 12 weeks.
I feel for your sister as it is hard, hard work to spend all day with a baby that won’t settle, but it will get better, just support her as best you can and hopefully things will improve quickly.