b4a/mysql on a tablet for your brother.
B4a on a tablet, as the client, with maybe local mysql on the client for temp storage,
then send it to a simple server with a MySql database server there. (can run on a windows machine)
B4a, can generate PDF invoices/reports
You can get a blue tooth keyboard working for the tablet if you really needed a keyboard for more input related tasks...
otherwise, Vb.net?
I would rather stay in Android/B4a code personally,
especially if you have the freedom to pick the language
for the system.