I have been a step-parent, and I have step-parents.
I have never received a card from SC, or given a card on Mother’s Day/Father’s Day to step-parents.
I actually find it quite offensive to suggest that just doing practical things like cooking, laundry etc means you should qualify for acknowledgment on Mother’s Day. In your own words OP you don’t really have much of a relationship with your SC as they don’t talk to you.
For me, Mother’s Day is about acknowledging your mum. And a mum is something infinitely more than a housemaid. In your own words, you barely talk to the SC. And you’ve only been in their life a few short years.
It’s no slight or disrespect to you OP. But you’re not their mum, nor are you carrying out the duties of one. I wouldn’t even say you’re on the role of an auntie as you don’t really speak to them…
It might be hurtful but maybe consider things from their perspective? They go to visit their dad and his newish wife, don’t really talk to her. They’re probably unaware of the exact dynamics of the housework but know they get fed with everyone else. They probably would be pretty surprised to think you expect acknowledgement on Mothers Day as there’s nothing to suggest that relationship.
If you stop doing things for them that you’d be doing anyway - cooking, laundry etc, then how will it work for your DC? Presumably your DH would retaliate in kind and would stop doing things for your DC, his stepchildren? Your relationship could really struggle as a result. Alternatively if you think that would work better, then that’s an option to consider and discuss.
I think if you feel taken for granted, that’s a conversation to have with your DH. You say he does a lot for your DC so maybe have a look and see if everything feels even. If not, he needs to step up.
I’m glad you’ve had a lovely day with your mum and DC. Try not to let this ruin your day.