I am having second thoughts about marriage

Basically I have a son from a previous relationship. He has downs syndrome and a few other concerns like irreparable holes in his heart for example.

He had a g tube for the first 2 years of his life and he still cannot walk (going to PT and he is thriving!). He will be 3 in September 2021

Anyways mu fiancé has always been fantastic and treated him like his own son (we started dating when he was 1 and we just got engaged two months ago)

My fiance has this female friend who I have never met but he is always very open so there is never any concern to be honest. For some reason the past 2-3 weeks she has been blowing up his phone and really taking a lot of his time and attention. She went through a break up nearly 6 months ago and all of a sudden she is having a tough time now. (((Can I also add this small fact. We got engaged 2 months ago decided to keep it to ourselves for a hot minute. So we announced out engagement 1 month ago on social media so we could have a second to enjoy it just being our news for a short time. Basically we announced it and then she reached out)))

She is always texting calling, crying, venting, needing his attention. And some of it is dramatic. Like she will post things on social media for everyone to see (because then everyone will ask what's wrong). She will be like today is my last day. And I hate my life. Or if I kill myself it was because of you (her ex). Or will take a snap of a knife on a counter and will be like lol. Or take a picture of her driving her car like 50 kms over the speed limit. She posts dumb shit!

Apparently she told him today that she is struggling so she took 15 Tylenols like three days ago and then regretted it and drove an hour to the hospital by herself and needed someone to talk to - that someone is him. And somehow she isn't being held in psych or monitored as a person who may harm themselves despite take a handful of pills.

Our son had a seizure today. I scrambled, got him to the hospital, called my fiance who said "I'll be there as soon as I'm done here with (her name)." As she is at a different hospital.

I was upset. Yes. Because I felt like he was choosing her over our son. I told him don't bother coming.

I'm not sorry either. I mean she has family. Friends. A new boyfriend. For the first time I thought to myself I can't marry someone like this. Like telling me moreless to wait til he's done and he'll come after??

Am I wrong?