toilet training is now underway

Toilet training a child takes a lot of time and persistance. During the day she should be constantly asking her child if he needs to go to the toilet. All children learn at a different rate though and if he's not ready, then you can't force him. Sleeping through the night is a tricky one. For the first little while, I would suggest getting him used to using the toilet during the day first. Once he gets the hang of that then do the night time thing. Always put pull-ups on them when learing at night. Make sure they pee right before going to bed and don't give him anything to drink for at least an hour before bedtime. There will be some accidents and you cannot get upset about it. When going out during the day, don't put any diapers on him and let him know that if he has to pee, then to say something because there are no other change of clothes and he'll have to walk around wet. (always bring a change of clothes for those accidents but don't let them know you have them.) If there is an accident, make him walk around wet for a little to show him how uncomfortable it feels, then get the clothes and make it look like you didn't know you had them and change the little guy. My daughter used to get upset to go to the washroom as she thought that she'd miss something by going. She peed on the couch once and I made her help me clean it up. She never peed on the couch again! Pull-ups are good too becuase they aren't as bulky as diapers and normally they can feel when they pee in it.
All your sibling needs to do is stay calm and persistant on asking if he needs to go to the bathroom.
Good luck.