I’d like to point out something that everyone seems to be skipping. Your daughter’s biological mother is the one who skipped out on Mother’s Day with her child. It’s funny how no one is calling her a cunt or an asshole or saying she’s horrible for not spending Mother’s Day with the child she actually birthed and raised. You don’t even seem upset about that.....but you are angry with your wife, who has only been a step-parent for 3.5 years and only sees your daughters 4 days a month. 

Have you considered that given the length of your marriage and the age of your kids, your daughters likely have never really seen your wife in a motherly role and vice versa? That’s pretty normal. I see it all the time in posts written by step-kids. “I have a mom/dad. I don’t need another one.” “My stepmom is always trying to act like she can tell me what to do.” “My stepdad needs to back off. I don’t need his advice.” Blending families when the kids are preteens and teens doesn’t work like it does when kids are young, especially if Mom or Dad are still fully involved. Parental relationships don’t develop as naturally, if at all. A “fun aunt” or friendly type relation tends to happen in healthy households instead. 

There’s just a lot of information missing from this post. What kind of relationship does your wife have with your daughters? Is she close with them? Has she made overtures? Have they indicated at any time they see her as a mother? Has there been serious tension or bad experiences? Has she been given a motherly role in your daughter’s life? Do you and your ex allow your wife to actually parent? Is she involved in decision making as equally? Would your daughter have wanted to spend Mother’s Day with your wife if her biological mother was available? Has your daughter ever even acknowledged your wife on Mothers day before? 

Depending on the answer to any of those questions, your wife could be being perfectly reasonable or a total asshole. Either way, it’s not particularly fair for you to expect her to “fill-in” because biomom is busy doing something else.