Challenge it, or take as much time as you need. My dad was like this. I used to get run ragged, doing 4 hour round trips at great expense, missing out on DC to be summoned to do all sorts of trivial things for him as if my time was utterly valueless except in his service.
I started to walk out of the room if he criticised. I'd just say ' Oh! I just remembered I have to...' and not finish the sentence. Sometimes I'd leave the house and go for a walk. But eventually, calling him out on it was the most effective respnse.
Don't do it emotionally. Say in a very calm, neutral voice: Mum, when you speak to me like that, are you aware you sound so mean and critical? Because it makes me not want to spend as much time helping out. Maybe the pain or the medication is causing you to be snippy but please make an effort to be kind and appreciative if you want me to continue helping you.
If she makes out your brother is golden-boy, calmly point out you do 3 times more for her than he does. Either jokily ask if she sings your praises as loudly to him, or just say, 'Oh well, if all I have to do to get your appreciation is turn up once a month like he does, I'll come around less often.'