The general concept is sound. However, your main problems are threefold.
1) The idea of Keiko secretly dating someone is a priori utterly bizarre and difficult to imagine given her personality, mental health, social skills, self-isolation and overall aversion to displays of affection, especially but not exclusively physical.
2) The idea of her dating Tenten is even worse, given that Keiko communicates verbally and loquaciously and is bad with anything else, while Tenten hardly communicates at all as far as Hazō's concerned.
3) Much of the evidence concerning Tenten (e.g. Keiko's reaction during the Neji debacle) is just as plausibly attributable to her being overprotective of the only friend she's ever made on her own initiative, and of her relationship with same.
These are considerably worse obstacles than when he was trying to understand Mari's position, and I think you'd need to hit Hazō with an evidence hammer to get past them, or at least bring him to a point where he has the conviction to ask Keiko directly. Not saying it's impossible, but mere accumulation of small details is unlikely to do it.
So he can tease out the unspoken details about a justu like shadow clone, but can't really figure out that his stepsister is dating someone not Shikamaru?