I think it all just depends so much on the individual people, the circumstances around it all etc.
dss is now here 100% of the time. He sees his mother for the odd couple of hours after school and she buys him a pizza, maybe once a week at most.
I don't have kids (she doesn't have more either) so he's not 'fitting into another family' really. And I don't do the majority of the care when he is here, dp does that, he's his father.
Obviously I help make the house run for all of us, do shopping, laundry (not his really, dp does his own and dss, but sometimes I do, or I hang it out etc), cook (again, in the week dp does most of the cooking, but I did the roast yesterday), am there for him to chat to (we helped him make his options choices, I don't think his dm even knows what he takes), remind him about his homework, take him out places, take him on holiday, welcome his friends here, give him lifts, help dp choose clothes and presents for him, take him to buy presents for his df and dm etc etc. dp and I share the cost of this too, while he pays his ex £500pm child maintenance (yes, I know!).
So, we are probably in a fairly unique situation. I think saying 'happy mothers day' and a small gift is fine. I don't think it should be expected (when dss came in with flowers I assumed they were for his mum and that he and dp had forgotten I had already sorted out his dm's MD present!) but I also certainly don't think they are saying "you are my mum".
This is the first time dss/dp has done it. It was very thoughtful of them.