Preface to say I have two very involved parents who I love dearly so this is not a situation where the step parent has basically been mum or dad because of an absent parent.
However my step parents are still a big part of me and are kind and loving additions to both my own life and that of my children's now who just see them as grandparents the same as any other because they know no life without them in it.
Therefore I always buy my step parents a little something on mother or fathers day. I don't get a card that says mum or dad on it but just a generic happy mother's/ fathers day and sign it from me and DC with a gift.
I think this is just a nice thing to do especially as my step parents don't have their own children so would otherwise receive no acknowledgement on these days.
So clear something up for me MN, my husband thinks this is could be seen as "disrespectful" to your actual parents (if they are involved obviously very different if they arent and you’ve been raised by a step parent) whereas I think it's just acknowledging someone else who's had a similar role to a parent and making them feel included on a day like mother or fathers day especially if they have no bio children as well. It's just a nice thing to do.
I am also a step mother to 1 child and never receive anything for mothers day, it's not something I would expect or be bothered about personally but just for context.