Hi Jakie,
I don't know how old you are, but I do know how tough being the eldest can be. And I also know how hard it can be to have adult responsibilities when you're only a child. My little girl, although a middle child, looks after her brothers a lot because they have special needs.
In the UK, we have an organisation called Young Carers which gives kids a break who have extra responsibilities within their families... maybe you could look into something like that with your mum.
And it's always worth trying to talk to your mum and step dad and explain that you really want to help, but sometimes it just feels too much...
Good luck, and hang in there!
B