tell your mom that although you appreciate the thought, have her ask you from now on if something needs tuning, in the politest way possible
as for your brother, if you have admin perms set up a separate account, or if he will bitch about it, configure the computer to have dual boot and use your own partition of the disk (and make sure it default boots into the os your brother uses). linux mint is a nice option for the alternate os. maybe keep anything steam/game related on the (assumedly) windows partition and make sure to keep all your good stuff on a usb drive if you have a lot of windows only programs