Some pretty valid points have been raised, my mother was subject to a similar situation as well as my aunty. The whole process ended up ripping both families apart, but there were other important details to factor into those causes. As someone with some experience from a similar situation, staying out of it and standing on damage control is your best bet, take your dad's word for truth and pay the situation no mind. If their conduct makes you uncomfortable, maybe bring that up with your dad, it's possible their doing something that they don't realize can be perceived as inappropriate or suspicious from an outside perspective, which could create problems for them if another party is viewing these actions.

When I tried to intervene in my mothers antics, I only got emotionally hurt by her. Her situation was a lot easier to deal with when I took the step back, looked after my younger brothers and dealt with the after-math the best I could. When I tried to get involved with my aunties situation, I ended up spending some time in jail for assault. Honestly, I can understand how you must feel, caring for dad and hoping for the best for him, but it's easier for you both if you just let it play out and deal with any collateral should it arise.