Desperate for advice/ parent seperated

Ladies who aren't with your child's dad:

I need your advice..

This is lenthy but please I'm desperate for advice..,

I have been with my SO for 2 1/2 years. From the get go I knew he had a son with another woman that I while heartily accepted. When we moved in together she hated it, and the only way he could see his son is to go to HER house or go with HER to visit. I still was ok with it, because it's his child. No we haven't went to court and this late in the year we won't be able to accomplish what we need. But since then we were able to let him come over for weekends. Then the beginning of this year we moved to my home town ( due to him losing his job and financial hardships)which is only 45 mins away from his son. She then wouldn't let him see him again. Now we are married, have a house, he has a great job, I have a job. She just let us have him a full overnight 2 weeks ago. Apparently we were informed that he has a event this weekend out of state in a military base. And somehow even though we told her that we both were coming she only registered one. ( I haven't missed a visitation since I met my stepson 1 1/2 year ago) and now I may not be able to go, after I specifically took off work.

He says I'm being selfish.

I'm young no kids of my own, non together yet. Shouldn't she be happy he's our focus?

So here's my question:

Why do women hold bad feeling so strongly against the other woman?

Is it wrong that I ask my husband to stand up for me?

If he doesn't go she will hold it against us for the upcoming holidays but if he does, I will be stuck in a house by myself after specifically requesting off for this..

Ladies any advice. I don't have a child of my own but I love that little boy like he was. The way his mom treats me. Any,advice helps.

Thank you