In this specific case what makes the discovery of a grandaughter of the author a massive surprise and plot twist is that her father and only child of the author was gay. Had he not been so, expectations on the existance of a living descendant of the author might have been different.
That's what makes his sexual orientation be relevant in this case: it explains why nobody expected there would be living descendants of the author and why her discovery was such a massive plot twist.
Had her father been, for example, a catolic priest (quite independently of sexual orientation), that information would've been relevant in just the same way and for the same reasons.
Whilst I agree that people's sexual orientation is irrelevant in most stories that aren't about romance (and no matter which way it goes, by the way), in this specific case it absolutelly is relevant to explain the behaviour and expectations of other participants in the story up to the point when the grandaughter was discovered.