There's plenty of information to be learned on the site.
Its Christmas so many are with family today but I am alone so wish your new chick a chance.
We just like to remind that so much can go wrong even in experienced hands & birds are not a business...they are lives.
We just don't want you, your family & the birds to have sorrow & it seems there has been enough of it. . Its something books have been written on & still they may not be enough.
We can only advise so much as you need to have a vet on hand when you are breeding as so many things can go wrong that will bring sorrow & are likely to cost more than any profit you could expect.
A little one has been born & hopefully the miracle goes well & there are no complications.
Give them a good nest box with good clean bedding...from what I have read & there are many threads on it so do some homework
clean wood shavings are a good bedding. The chick needs to stay very warm so hopefully the parents can provide it & feed the chick.
I would not disturb them too much as that could stress them & unfortunately if they stop taking care of the chick its not going to be good.
You could start planning for that...brooder, formula, syringe, heater, etc but you would also need to feed every 2 or so hours except between 12A & 5 to 6 AM. Did your sister have all the necessary equipment as she was "breeding" birds such as brooders, incubators etc?
You really should remove any other eggs (boil, cool & return from the other pairs or replace them with dummy eggs.
We understand this happened but it does seem you are inclined to make a business out of breeding these birds for a young niece.
You have to ask yourself is that what they really want or need? They have an unmentionable loss to deal with already so
they need some stability & shelter from avoidable hardship & loss. We have seen so much sorrow brought about by inexperienced
allowing birds to breed. We can't support what is likely to bring about suffering to yourself, your family & the innocent birds.
Just because you want to jump off the bridge to learn how to swim does not mean we should not advise you against it.
Breeding birds takes alot of time, money & is never really profitable so is not a good business. Most breeders do it as they just want/love to but if you think its something to do just for money its not likely to work for you. It's a 24/7 lifetime commitment with no time off & not something to pass on as a legacy to another. Go slow or you can quickly get in over your head. We see way too many Sad Updates to baby bird sick threads. You had have alot of sorrow in your lives & not wishing you any more.
There is a little life now & I sincerely hope it goes well. I wish you & the birds the best.
Nest boxes do need to be cleaned & bedding of aspen wood shavings or pine used.
Chicks need to be warm & depend on parents for that but the higher the room temp the better they will be.
Try to provide nest boxes that are easy to clean & avoid unnecessary disturbing the parents.
Good Luck