She has been a great single mother to 5 children and did her utmost best to provide for us all
I've always been far maturer than my age- I've been cleaning and cooking since 8, I was trusted with large amounts of money to bring to the bank, paid bills and did the food shop for then 6 people and came back by cab from age 11 & up.
Well those two statements contradict one another. She hasn't been quite as great as she's trained to believe she has.
When I hear someone say they were behaving like a little adult as a young child I see a child who was expected to behave like an adult as a child. That's is not good parenting.
My teenage brother still lives at home & so I will be taking care of him also. He is a legal adult age working on his future and only recently started part time work that doesn't pay a huge amount
And now she wants to bail on her responsibilities to your brother, who may be 18 but he clearly isn't ready or able to be financially independent yet, and foist the burden onto you. All the while making sure she keeps her home so she can swan in and out of the country as she pleases. 
While you could easily leave and rent your own place or share with a friend, I think it would be awfully unfair on your brother if you lost your council tenancy.
At only 18 he will not be earning enough to rent anywhere decent and could end up in a house share situation which is rarely as fun as it sounds unless you are at uni or renting with young professionals.
He's likely to be pushed towards some really dodgy people in grotty places, or a hostel fully of druggies and alcoholics and people with mental health problems, or end up homeless. That's the reality for young men with no family or financial support. As a young single person with no decent income and no children it's very, very hard to get decent, safe housing.
I think you need to do everything you can to keep hold of your council tenancy. Go to CAB and ask about your options. Could you consider keeping your tenancy but doing an exchange for a smaller place? That way the two of you keep a roof over your heads but don't have to pay costs on a bigger place than you need. And your mother and her husband will have to make other plans when they visit. I know it will be a shame to lose the only home you've ever known but in this case it seems like the best solution.
It would be great if you could stay where you are and sublet the two spare rooms but I doubt its legal and do you want the stress of being found out and punished for it?