These types of things will pop up from time to time. You’re human, just trying to do what you think is right. Your husband’s ex is human too, and sometimes gets her feelings hurt. You can’t control what upsets her.
I can tell you have the best of intentions and are trying to do the right thing. I’m sure at least some of ex’s reactions are because of jealousy. You and hubby have been married less than a year — she is still adjusting to her kids having a step-mom also.
In personal experience (very similar situation to yours) a lot of this “small” stuff turning into a “big” deal dies down after you’ve been married for a year or two. Trust me, I get how frustrating it can be, but you do you! I once had a conference call with my husband and his ex because she was furious I called her by her first name and not “mom” or “mommy”. I was so caught off guard and confused — Why would I call her mommy????
Hear her out, make reasonable changes, but don’t let it question your relationship with those girls. They are your kids, too!