Well, I have the same living situation. 50/50 custody with my ex with an 8 year old son. I am remarried for 4 years now with a 20 month old and ex is remarried with a 1 year old. None of us are great friends, but we get along.
Sounds like the ex's wife is jealous to me. Why is she butting between your ex and your communication? Are you texting him too much perhaps? I think you need to not respond to the step-mom at all in these instances. And I would limit communication with ex for awhile unless it is absolutely a must. Let things cool down. The step-mom is probably feeling like her feet are getting stepped on, but she needs to take that up with her husband and not you. Your ex is probably forwarding your texts to her and letting her handle things, and she is probably very overwhelmed with a new baby and feels out of control. I agree that you should save all of the bad texts and emails incase this continues.
Here is what I do: I never deal with the step-mom unless I have to. I talk to my ex all the time (once or twice a week)and we email or call back and forth about things to keep things consistant for our son. I have found that it is better to pick up the phone and talk to him at work or on his cell. The emailing and texting does not often go too well and it is all too easy to do it too much. I have found that it is best that the stepmom and I do minimal communication. As long as I have a good relationship with my ex, he will take care of his wife. And I know she treats my son really well, so I don't get involved with her.
Also, why isn't your husband involved? My husband has been in my son's life since he was 3 and he is a huge force in his life. My husband has a say in what goes on in our home and I have taken his advice many times. My ex has the same approach with him - minimal contact and deals with me, then I work things out in my home. I think he has to step up to the plate and get involved with your son and parenting him. Even if i don't agree with him all the time, I never cross him or go against him in front of my son. (I get on him in private if I don't like how he is handling things.) He is the head of our household and deserves a say in what goes on with everyone in it.
I have a had a problem in the past of trying to communicate too much with the ex and it has back fired. It was out of the same concern for my son's consistancy, but it was also hard for me to let go of control over my son's life when he was away from me. I still really have a hard time with it - even after 6 years apart from my ex. Make sure your expectations are not too high, as well. It was really hard for me (and still is) when things are not done like I think they should be at the other house. As long as the main things are agreed upon, that is all you can expect. I basically tell my ex what we are doing at our house (bedtimes, vitamins, routines, etc.) and let him decide what he wants to do in his. As long as there is not a HUGE difference between the two, your son will be fine. My ex lets my son be on the TV/computer way to much and we have limits, they eat in front of the TV and we eat at the table, behavior tolerances are different, etc. Your son will adjust to the expectations as long as there are clear rules in each home - even if they are different rules. Is it ideal if things are exactly the same in each home? Of course! But that is just a complete impossibility. If you and your ex were still together you would have parenting differences and such, which only seem worse in a situation like this where what you say and feel really has no bearing on the ex - you have no influence over him anymore.
Talk to your ex, if after you really cool it on the communication, she is still coming after you. I bet she calms down soon if you take a step back and let her run her household the way she sees fit and have some time to adjust to being a new mom.
Best of luck!