I think it depends- has op’s mother even asked step-dad to help? Or has she decided it’s easier to ask OP than him, or thinks men shouldn’t do that kind of thing, so her default is OP.
I ask because I had a not too dissimilar situation in my own family- my aunt was looking after an elderly relative, and she seemingly could not ask any male in the family (her husband, sons, brothers, nephews) for help but would contact her female relatives- my mother, myself, her other nieces, her DIL’s without any concern whatsoever. My cousin asked her why she had contacted his wife rather than him to ask for help, and she replied that she didn’t think men should do that sort of thing and didn’t think they would want to help with personal care.
Perhaps OP’s a bit put out because her mum hasn’t even asked her own husband for help but wants her to go round daily, whilst he lounges in the garden? That is really not ok for her mother to just accept that, but expect more from OP.
it’s also possible that OP was not consulted about what help she could provide before EoL care at home was decided upon, yet is being expected to step-in (and to do so daily, it sounds like). She may also have her own commitments which are not being taken account of.
Of course, if op can help it would be the right thing to do to do what she reasonably can. But if her mum needs more help than she can offer, her mother will have to ask other family members to help. Or make alternative care arrangements.