Funny how the most popular pairing for any sister in touhou is incestuous. In the Aki's and Tsukumo's cases they are probably the only pairing.
Beats the Prismriver's pairings of "Wait, who are the Prismrivers, again?"
It's the curse of having characters being naturally put into a set of two. Shippers tend to just look for the easiest pairs of the two characters who share the most in common, and if DNA happens to be part of that, then so be it!
The Tsukumos may just be in an early phase where they haven't yet had any popular pairings, but they do seem to be heading down that road, as they're the least popular thanks to not being notably tied to the plot, having any particular relation to anyone else, and *sigh* being music-related, which is apparently to Touhou what a weather villain is to superhero comics.
The Akis... well, I do see them in some odd pairings. There's the occasional Reimu/Minoriko "food" pairing. You sometimes see someone like Yuuka because of odd things like Rappatu's work, but yeah, they're mostly there as butt monkeys.
