I'm a step mother - what I wish I could say to the mom
Child Support - I don't care that my husband pays child support for his child. That is his responsibility. But do not sign your child up for an activitiy, without first discussing it with my husband and expect him to pay for half of it. Also expect him to say he has to check with his wife. I am his wife and we are running a household together. This means that we need to see if a new expense will fit into our budget. If it pisses you off that he has to check with me, then I guess you can pay the whole fee on your own.
Visitation - Your child is his child and is always welcome in my home. However, if my husband cannot take your child, please do not expect me to drop what I have going on to babysit. If it's possible, I will do what I can - but when your child spends every Monday with you, I am free to make plans for Monday.
Activities - Yes, your childs soccer game is extremely important. However, my child's surgery is a little more important, considering this is a surgery that will save his life and not just getting a cavity filled. Sometimes my child will take precedent over your child - that's what happens when you get divorced and one or both parents have more children.Sometimes, your child's activities will out weigh my sons. Like if your childs team makes it to the championship - my husband sure will skip my/our child's pre season football game to attend your/his child's championship game.
PTA and other things of the sort - I joined the PTA because I have a child that attends school at this school. You joined for the same reason - sometimes that PTA will recognize my thoughts and opinions and sometimes they will recognize yours. If you don't want to be on the same PTA as I am - take your child to a different school, because I really couldn't care less that you're in the same room as me for a few hours a month.
Field Trips - Yes, my son and your daughter are the same age, same grade, same school. Sometimes their classes do things together, like class room parties, and field trips. I'm an involved mom, so I'm going to volunteer as much as I can - You are also an involved mother, volunteering as much as you can. If your daughter asks me to refill her juice, it's not me taking your place, it's me being a parent volunteer in the room. If my child asks you if he can go to the bathroom, be kind and let the kid go to the bathroom like any other parent volunteer would. Don't take it out on my kid, because the teachers decided to have a joint party/field trip/ movie etc.
I have accepted that my husband has a child from a previous relationship - maybe you should accept that your child's father is married and has a step child and an additional bio-child. I really have nothing against your child, or you and your existence. However, I exist too and sometimes we will be in the same room