I understand why you think it's cringe. While excess is always bad, I think having a game that uses and takes advantage of taboo talk is good.
I mean, how many games (especially western ones) use patches that just change the word "landlady" to "mom", "tenant" to "son", etc. And it's pretty tiring to read the same phrases every time. "I can't think that way about her. She's my landlady mom, for God's sake!" Or "We can't do this, you're my tenant son and I'm your landlady mom!". It's ridiculous.
That's why I think having games like this that actually make use of the taboo factor in conversations is a good thing. As you say, maybe it's a bit excessive in this game and I agree that some of the phrases seem a bit ridiculous to me lol. But I think that's what is meant to be achieved. Use the word "incest" in every way possible until it seems ridiculous.