Hello and welcome!!
Just a few tips for you:
Be sure the kitties have a room of their own for the first week or so, keep them isolated from the family for at least one week and then slowly introduce the house putting them back in their room over night. This will get them acclimated to your smells, sounds and schedule and will help reduce and stress of the new environment that might cause marking or soiling of your home.
Be sure you have three litter boxes and maybe put two next to each other. One for each cat and an extra for marking or pooping. You may find they like doing different things in each of them! Be sure since you have a child that these boxes are in secluded areas of the house so they have their privacy. Unless you have a toilet that is not used very often I would recommend that you not put their litter boxes in a bathroom. This can be very stressing to some cats as if it's shut off when they need it or if the human smells or cleaners you use may put them off using it.
the use of Feliway may also make the move a little more settling for the both of them. You can purchase this online or from your vets. A few scratching posts scattered across the house would be useful too for marking and stretching. They tend to like them near doorways and at the tops of stairs.
I would recommend sticking to the diet they are used to and also the litter. It might be worth keeping some of their old litter from your brother's house to help them know it's their new home. sprinkling this into the litter boxes you will use round the house. Don't be surprised if they don't eat very much or hide. they may also not use their litter trays at first for a few days! they will settle with time.
Be sure you give them some high places to sleep and lots of activities! They love attention and to play. The more play time they have the less destructive and acting out behaviour they will do. Keeping a routine will also be very helpful for this. I would leave bedrooms off limits and your son's room off limits to the cats for at least one month. Then eventually introduce these rooms with play time and treats. Don't just leave the rooms open for exploration as this can sometimes set off marking and soiling of beds and items in the rooms.
Be sure their water is separate to their food areas as most cats don't like drinking in the same place that they eat. Also make sure they have their own food areas so they don't feel uncomfortable in their new home.
hope this help and congrats on the new kitties and baby to come!