We had mice in our loft and have loads of humane traps. Unfortunately bad news is, it takes ages to get rid of them unless you can find where they are coming in. If the 'mum' has a litter you are looking at around 6 at a time and she will give birth usually 6 times a year. Busy animal!!!
Make sure when your bro lets them go it is FAR from the house. We usually go about a mile down the road sometimes further, ppl recommend just 1/4 of a mile but I make sure we go further and round corners so they cant make their way back lol
If you let them go too close they will keep coming back. Also put all bunny food etc in plastic containers with lids on so they can't smell it. We also discovered mice in our shed, where we kept the bunny food, so it all went in containers, but they go for anything, even things like soap. It is mad. The best bait i have found is peanut butter btw, that catches them really quick. the ready baited traps are 'ok' but peanut butter is better IMO
I would also advise to check the traps every few hours, because they do get stressed, and start chewing the traps, and then let them go as soon as you can as not fair to keep them as it will scare them and they may end up dying.
I have done loads of research on the whole mice issue as its a big thing when they come into the house, and killing them was a complete no no for us.
I hope you manage to get rid