Wow, I have been through some of this with my son, so here is what I can say:
Legally, I think his new wife may be considered a relative, but do you all really want to go to court over this? A judge will probably appoint you to mediation to discuss it and come up with your own solution anyway.
1)What is your main objection? If you changed jobs specifically to be home when the kids get off school and your ex was unwilling to make the same change (still working late) that may count in your favor. I think since you are making the work change, your request to be home with your kids until your husband can pick them up is valid.
2) on the other hand, things change. jobs change, people get remarried, schedules change. Is your objection just that you want to spend that time with your kids yourself? Or do you have some objection to the new wife?
3) What is her current relationship with your kids? Are they comfortable with her? Do they like and trust her? This woman is potentially going to be around a long time and you will have many school conferences, sporting events, parent nights,etc. where you may have to be interacting.
4)Do you feel that she is a safe and responsible person who is good to your kids? That is a more important question that do you like her or not. You don't have to like each other- you just have to be polite to each other and get along well enough for your kids.
When my ex moved in with his now-wife, I actually called her and asked her to meet me for coffee. We talked and I asked her what her expectations were of her relationship with my son and if she was prepared to have him in her house on the weekends, etc. ( his dad moved into her condo). I asked about her 'house rules', pets, would he have his own room there, etc.
It turned out that she had come from a divorced family and said that her main concern was to keep everything as stable and happy as possible for my son. She has kept to that, and even though we don't always agree on everything, my son actually sees MORE of his father and they are more involved in his life- and I am sure it is because of her and her background.
So we aren't best friends, but we have an understanding. She actually never comes and picks up my son on her own without his dad, but if she asked to, I would allow it.
I think it is totally reasonable for you to explain that you got a job specifically so that you could leave early enough to be with your kids afterschool and that is not something that has changed. But maybe work out some other time she could pick the kids up? What is it that THEY really want? It your ex trying to make up for the fact that he won't leave work earlier or is his wife trying to get some bonding time with the kids? If you can, talk to her about it- she should understand your need to see your own kids as much as you can, unless she is totally unreasonable. Of course, if you think she is not a safe driver or responsible, etc. that is a whole other story. But don't be threatened if all she wants is to get to know the kids better- trust me, that is a good thing and you will still ALWAYS be the mommy. THAT doesn't change!