I will say again, It is 'mother's day'. The kids get something for mother's day, you will find that most 'dad's' or 'men in mother's life' buys her something on behalf of the kids, and/or from him.
If she is remarried then you don't have any responsibilities to anyone but those kids. If THEY want you to help buy her a gift than it is up to you to say yes or no.
I think it is highly rude of her to expect you to pay for a gift for her. Yes, she is your kid's mother, but she has another man in her life and it isn't as if kids don't make things in childcare and school these days.
If mother's day to her is about what 'bought gift's' she get's, then she needs to rot in hell
As a mother myself, I would probably throw back any presents my ex got for my daughter to give me right back in his face.