being a step parent sometimes mean that in certain circumstances (in established homes and families) you do end up doing lots of "parenting" things. I certainly do. Not to replace dss's mother but just because she isn't present when dss is living at our house.
....but his Dad is present! Why are you doing those parenting things, not his dad?
I've spent hours helping him write his name, talking with him and my (older) dd about school and what happens, Ive sat and seem all his little name labels in his uniform
By doing these things you have replaced the role his Dad would play if you were not together. And if his Dad has asked you to do them instead of doing them himself - then perhaps his ex is asking herself whether he is as committed to his DS as you are?
when the DC is with the father the stepmother will be in charge in her home, she may well be there alone with the DC and the one putting them to bed etc.
If a parent has shared care, but delegates that to a step-parent due to work or social commitments, or just personal preference, then the care arrangement should be, imo, reviewed.
Caring and parenting our DD is the responsibility of me and her Dad, and if I am unwilling or unable to do so when she is in my care, then the person to whom that responsibility falls is her Dad, not a person I choose to delegate that responsibility to without prior agreement of her Dad.
One off, emergency arrangements are one thing - but if a step-parent is routinely relied on to do school pick-ups, overnight care, homework help and listening to reading, then it raise the question of whether the parent can actually provide their DC with the level of commitment required for the care arrangement in place.