Indeed, as @Hankunamatata just posted
it boils down to her dp. He needs to step up
In an ideal world, the SM would never have been roped into regular child care in the first place: that's what the DC have their two parents for. Absolutely right for the SM to consider the DC part of the family and to provide cover for the parents in an 'emergency'.
Doesn't sound as if this is the case here.
SM has been people pleasing, not setting good boundaries and picking up a lot of the slack, ending up doing a lot more than her fair share.
It doesn't surprise me that her partner is trying to guilt her into continuing to carry his parenting responsibilities.
While it is entirely reasonable for the actual parents to step up, we are left wondering whether anyone could have predicted this outcome and that the time to negotiate any changes in responsibilities was long before conceiving, never mind when the baby actually arrives.
It would of course be entirely wrong for a new partner to 'prove her value ' with hands on parenting and then withdraw her labour as soon as their own DC turn up.
The question here is why was the SM doing the role of a full parent in the first place.