When it's Shikadai's birthday, Kankurou never fails to show up. "How sweet" everyone says, "he must really like being an uncle", which is a correct assumption. He arrives the day before, brings Shikadai the coolest gift and plays with him until the kids show up for his birthday party. That is, when Inojin arrives - because it's rare for someone to arrive before Inojin. Shikadai runs of to meet his friend and to show off his awesome new toy. Everyone assumes that it's because Shikadai would rather play with someone his own age, but they're wrong there.
After all, Shikadai inherited his father's brain. For as long as he remembers, it's Sai who comes to drop off his son, while for the birthday parties of his other friends, he leaves this task up to Ino. On top of that, all the parents drift off as soon as they've delivered their kids and made small talk with the hosts, but Sai stays. As soon as he arrives Kankurou's attention shifts fully to him. They seat themselves on the couch in the most private corner of the room and barely even look at anyone else. They hardly touch, apart from their knees and the occasional there's-some-wipped-cream-on-your-face-here-let-me-get-it-for-you, but Shikadai knows the way they look at each other from his parents.
Once or twice Sai brought a sketchbook full of designs for Kankurou's facepaint and Kankurou stated loudly that he wanted to "try them all out right now". He dragged Sai into the bathroom, where he keeps an extra stash of facepaint, in case he stays over at his sister's. When they emerged again after an hour or so, Kankurou wore a fancy pattern that was way too complicated for daily use. Shikadai had seen a hardly noticable purple hue on Sai's lips and he doubts it was there because they tried painting his face as well.
When Temari celibrates her birthday (which is every year) and when she's able to force Shikamaru to throw a birthday party, it only makes sense for Sai to come accompanying Ino, so he and Kankurou - who of course also always comes - keep a little more distance on those occasions. That is to say, there's a hands-width of space between them, they try (and fail) to look at other people besides each other and don't wipe off each other's faces.
Shikadai knows both his parents know that there's more going on between the two, since Shikamaru is probably ten times more perceptive than he is and it's not likely there's anyone who knows Kankurou better than Temari. Even so, they act as if they don't notice and leave Sai and Kankurou be as much as possible on Shikadai's birthday.
He may be clever, but he doesn't understand all the secrecy. Why would Sai pretend to love Ino, when he actually loves uncle Kankurou? He also doesn't get that his parents don't interfere. Ino has been his dad's friend since forever but he keeps her in the dark about her husband's actual affections, which Shikadai assumes is pretty unfair even if she already knows about it (and Shikadai can't imagine that she's that blind). Kankurou is his mum's younger brother but she does nothing to help him progress in this currently dead-end relationship that can't possibly make him (or anyone else involved for that matter) happy.
It's probably some troublesome adult stuff and he's not bothered enough to ask anyone for an explanation.
Frick, I keep thinking about these two.. Sorry it's so depressing, I just thought about what would happen between them when Sai married Ino and yea... I'm not convinced Temari would actually do nothing about the situation, but let's say for dramatic effect that she doesn't.