Need advice on my dad and his girlfriend’s relationship.

Hi this is might be a bit long but I’m 21 F and my dad met his girlfriend when I was 14 I watched their relationship blossom and I could tell how happy they made eachother even though due to previous girlfriends trying to get me out the picture I gave it a chance and it was good. They’re both drinkers not alcoholics but they like to drink at the weekend just like most people. Fast forward 6 years (2024) they have 2 children together, a lovely home and everything they need to create a beautiful family together. My dads previous relationship he had been in had been 8 years deep 2 kids deep and he was cheated on to which he moved out and tried to start living again even though this was a heavy heartbreak for him. I moved out at the start of last year and returned this week but I returned to them being in absolute ww3. My dad is so convinced his girlfriend is cheating on him any slight thing like I went out yesterday and he asked if she asked when I was going out and then after a couple beers or whatever it all spiralled into who is he you’re cheating on me blah blah blah. It doesn’t stop, I’ve asked her why don’t you talk about it sober and she says he says he doesn’t think she’s cheating which must be a lie because why is it brought everytime they drink. Speaking to them both separately I can still see they love eachother as much as they did when they first met but it’s honestly heartbreaking. They’ve also have physical alterations over this and it’s just horrific to see and I feel so trapped because it’s not my relationship and I’m years younger than both so who am I to go give them advice.