How do I deal with my mother's savage comments??

I am not going to tell the whole story here (there is really nothing to it, this is ALWAYS out of the blue when she starts talking like this)

I just want to know how any of you would cope with certain comments like this:

My little sister says "I love you Mum"

Then my Mom says "at least someone does"

"You watch I might not be around one day"

I tell her I am doing something to help her and she says "it's not like you do anything around here" or "I am all on my own you do nothing"

The list goes on..., I don't know how to react to stuff like that without wanting to rip my hair out or cry..

We have a strained relationship and I try my best to respect her and be a good daughter but she makes it hard.

What would you say ?