As a secondary school teacher, I think she gets at >10K+ salary. Add up with your brother's, should have >30K+.
So if they marry, they should move out - to buy/rent. Your mum should refuse to live with them.
Honestly, if your brother really loves her, no one can stop them to get married. But just your mum should protect herself by not living with them. Or, you mum should refuse to do any housework for her, tell your mum BE STRONG !