

I do get the discomfort but what are you “moral” objections? Is it that she too immature to make a decision? Or is it that you think your dad is taking advantage of her?
It’s worth working through why this is a moral issue for you, you’re a bit vague about it. In Europe the age of consent is variable but 16 is common, and it can be a bit jarring when you see the reactions of Americans to anyone under 18.
But in Europe adulthood has generally begun at 16, including being able to leave school and work in many places. The voting age is even being extended down to 16 from 18 in some places. So it’s not as clear cut that someone at 16 is not able to make independent decisions as American users sometimes make it seem.
Having said that, I personally don’t like the idea but more pragmatically for the age difference and the maturity difference. She can consent but there is a very significant change in maturity from 17 to 25, and I’m not sure how viable a relationship someone who is 48 can have with someone who is 17.
I think they are both adults and of the age of consent. You can express your concerns to your dad but ultimately it is both of their decisions and you should stay out of it beyond that (unless there are other issues that arise). I wouldn’t go too far judging him beyond that - he will be your dad for the rest of your life. If you had a best friend who was 17 and in a relationship with a 48 year old, you might express your opinion but would you interfere beyond that? Probably not - this should probably be the same.
Perhaps but what is the value of human life? Being remembered of living and enjoying life? You won’t be around to care that no one remembers you, but you are here to enjoy life right now. So why be bothered by what people in 600 years are up to.