First of all make sure from now on the school sends all correspondence to both you and your ex-husband. Schools do this all of the time now. I was hoping you would say her wrote down 3 because he had included you, but just forgot to tell you.. A 1 year old sits in a parents lap so why would they include the infant in the RSVP?
I understand that you feel insecure and alone in these situations, but remember that is your choice. You can feel good about the fact that you love your daughter and she is obviously very sweet and very intelligent.
You do not need another person to make yourself feel good about yourself, only you can do that.
You will never be able to change this girlfriend/new wife, so figure out what it is you need and let her know. I know this is easier said than done, but as a child of a terrible divorce, use your daughter as your inspiration for getting along and always taking the "high road.".
Approach her with thanking her for being so devoted and dedicated to your daughter and then lead into, "may I ask your for your help with".. .. Let her know the 3 of you are now going to be a team, so it would make it easier if you all could keep an open communication about all thing with your daughter,. Let her know you will do the same for them. Then the 3 of you need to figure out a way this communication will work for each of you.. Texting, email? What is the best way to keep each other informed.
Decide how you want to be informed about emergencies, accidents, school related events, teacher conferences etc.. Make sure that you all are clear and encourage everyone to share anything you think the other person may want to know even if you do not really care, always thank them for any info they share with you. ..
Remember now that they have a sibling for your child, the stepmom will always be in your daughters life.
I am sending you strength and patience..