I agree with Ann-m
Do have a look at the Living with Secondary Breast Cancer part of the forum - there’s plenty of us on there living for many years with secondaries (or mets as we tend to call them). There is a very active thread called ‘Bone mets please join in’ which most of us share our thoughts and experiences on. We are a supportive bunch and happy to help with any questions. If it helps the initial advice would be don’t google your Mums condition - the ‘facts’ about survival rates etc are way out of date and scare the pants off everyone! I’ve been living with bone mets for over 10 years now so I should know!
Sorry to hear about your mum.
Your brother is right to be optimistic, in many ways, stage 4 cancer is moving into the mangement of chronic disease territory & I understand, is increasingly referred to as 'chronic cancer' now.
There are certainly quite a few ladies who have lived with secondary bc for many years here.
Do also have a look at the 'Living with secondary breast cancer' part of the forum & do post there as well, if you feel you need to ask anything, we're all happy to help.
You sound a wonderful daughter, btw