AITAH if I blow up my dad’s secret relationship by telling my stepmom?

So this whole thing has spiraled over the past month and I’m still kind of shocked, but I’ll try to keep it short(ish).

Before Father’s Day, I talked to my dad and he said he was going off-roading with my sister’s fiancé. Last minute, he canceled and said he pulled a calf muscle and would be staying home. Weird since he usually pushes through pain for the sake of doing fun things, but okay. Around the same time, my stepmom and her sister told me they would be out of town for the weekend and to “show my dad lots of love” while they were gone, and that they felt bad they had to leave him alone that weekend and stuff.

I didn’t go see him. Long story, but I don’t have a close relationship with him. I sent a "happy fathers day" text and left it there.

Turns out, my brother did go over. He called my dad beforehand and when he got there, my dad had made a pasta dinner for two. There was a woman there, over a decade younger than him, who he has never seen. My dad said she was a "very" old friend from work, that they have known eachother forever, but… we’ve literally never seen him with a platonic female friend in our lives. Not once. Whenever he's around women, it's either his partner or his friend's wives, partners friends, etc, and he rarely talks to them.

Fast forward: I text on 4th of July saying happy 4th, and he says he’s out of state at a family friend’s house, sort of. A house of someone I’m MUCH closer to than he is. He doesn’t even like them, so I wonder to myself what's up, but don't think to deep about it. I brushed it off. But it gets weirder.

My sister calls me a couple weeks later. My dad had called her and told her he was going to a campsite 3 hours away "just to take pictures and sit in nature." I cannot emphasize enough how much this is not his vibe at ALL. He said he was going alone, then complained about his wife being boring and never going anywhere with him. My sister kept repeating “well be safe out there alone…” until he finally admitted he wasn’t going alone; he was going with “an old friend,” but didn’t want to tell his wife because it was a woman.

Then, right before this trip, he calls and asks me to watch a labradoodle that belongs to a "friend.” I just know it’s the same woman’s dog, too. I ask if the dog is good with other dogs and birds, and he casually drops that they brought the dog with them to that friend’s house for the 4th of July. Which his wife does not know about. He explicitly told my sister that he went alone as well. I agreed to watch the dog because I wanted to meet her, see if I just forgot someone from the past or something, and get a sense if this is really a big deal.

They show up today. I have never met this woman. She’s probably 15 years younger than my dad, and when I ask “So where are you headed?” she freezes and just says “Oh… uh… just have some things to do…” L i a r. I'm pissed.

I also texted a mutual friend who was at the property on the 4th. They confirmed my dad brought this woman, and said she was awkward, claimed they were “just friends” even though no one asked, and that my dad told them not to tell anyone she was there.

So: I now know that my dad brought this woman on two solo trips, had her over for dinner on Father’s Day while his wife was out of town, introduced her to people while saying “don’t tell,” and she’s actively lying about where she’s going now.

I’m 99% sure my stepmom doesn’t know any of this. She's so sweet, and she and my dad are supposed to move out into a new home in less than a month. I have had conversations with her and she always says she never worries about my dad cheating, he's not that guy, that they are just. so good together and that's the last thing she worries about. She says she likes not worrying because she doesn't like the person she becomes when she has to worry. This has been keeping me up at night because I know in my heart I would have already told if it were anyone else, but I have this tugging feeling that since it's my parents, it's not my place.

Would I be the AH if I told her? I'm scared he's going to blow up on me. He's sort of the aggro, narcissistic type and I'm just really nervous.

If you told, would you go straight to her and lay it all out, or would you try to be more discreet in helping her discover the truth? Thanks in advance. This is all really stressing me out.