"If she is good to them, allow her to be at the children's extracurricular activities. You don't have to sit next to her, or be her BFF, but to just allow her to be there. She probably feels some bond with your children and wants to participate in things."
I don't know about this statement..."allowing" her to attend things IF she is good to them. I think that's a bit outside biomom's scope of control, no? I feel that kind of attitude would put this new relationship right into the toilet. Whether or not she attends activities is between stepmom and dad. Biomom shouldn't be giving her permission if stepmom behaves....sounds condescending to me.
My own advice: Remember you're only getting half the story (and we're only getting half the story, too). When your daughter says "stepmom showered with me while Daddy played on the computer," it doesn't mean Daddy is neglecting his daughter's care or allowing his wife to do inappropriate things. In my own house, it means "I asked ProtoLawyer to help me rinse the shampoo out because I can't do it well yet and I'm getting uncomfortable with boys, even Daddy, seeing me in the shower."
(Now, that doesn't mean ignore your gut if something seems wrong...it just means be careful about jumping to conclusions based on little information. )
Also, about that "neglecting parental duties" thing--my partner's been on the receiving end of that accusation a few times, mostly for things that just work a certain way in our house. In an "intact" family, people have different ways of dividing the work, and the same is true for a stepfamily. For instance, laundry is handled here kind of ad-hoc...whoever comes across a load pitches it in the washer. I'm not going to sort my SD's stuff out because it's "Dad's job." Segregating stuff related to SD's care would cause way more work for everyone. Also, when SD's mom heard "Proto went to Walgreens to get my medicine" from SD, she heard "Dad's not doing his job." In reality, I was taking care of the prescription so Dad could comfort the sick child.