Help - baby daddy’s partner is overbearing!

I’m sorry this is a really long post I had to get the rant out!

So a little background I have a 5 year old son, his dad and I are no longer together - We’ve had a lot of ups and downs but finally after 4 years managing to co-parent amicably and friendly for our son’s sake. We’re both in new relationships which are great my partner gets along with him and I get on with her it wasn’t awkward or uncomfortable at all.. until recently.

My new partner and I are due our rainbow baby girl in January - and my sons dad and his new partner are also due a baby in January! When we first found out they were also having a baby we thought it was funny and unbelievable that our son would have two different siblings at the same age. At first everything was fine but now she’s just waay to overbearing. We are 6 days apart she’s ahead of me and I just feel like she constantly steals my thunder. I’ve not once enjoyed this pregnancy yet as I feel like she’s got to have all the attention. Her social media everyday has a post and there’s always some sort of “complication” because her first daughter was premature. She’s desperate to have her baby in November it’s her constantly messaging me this and that about her appointment here and there and whys her baby measuring bigger than mines? and whys she a “whale” with a big bump and I hardly look pregnant? She constantly makes me feel shit and she’s starting to grate on me!

She’s also started pushing my son aside now she has a baby coming with his dad. This is what’s annoying me the most. My son is my world, my first born and I’ll never watch him suffering at another woman’s hands.

She used to dote on him and I couldn’t fault her she was the woman that took care of him when he went to his daddy’s but now my son comes back disheartened and feeling left out because she’s all about her little girl and their baby. He keeps telling me that he can’t wait for our baby to come because they will be best friends and he will always protect and look after her which fills my heart as I’d never want him to feel any less loved when his little sister comes along. But when I’ve asked him about his other baby sister coming at his daddy’s he doesn’t really say much about it as I know he’s not as involved there like he is here. I just don’t know how much more of her shit I can take before I go ape shit on her ass!!