It was mentioned in episode 1 of Ufotable's Fate/Unlimited Blade Works that Tohsaka used to go to the archery dojo to watch someone. Was she secretely watching Shirou because she had a crush on him (but how?) or was she looking out for Sakura? Did Tohsaka know of Sakura's feelings for Shirou?
I would assume it was because of Sakura. She knew full well of the dealing between her father and the Matous, and it's revealed in Heaven's Feel that despite Sakura's changed appearance (due to the brutal abuse Sakura suffered), she still knew Sakura was her sister.
For comparison's sake, this is how different Sakura looks before and after she was adopted.
(Left - Pre-adopted Sakura. Right - Post-adopted Sakura)
Also Rin turned up at the dojo after Shirou left, so we can rule him out. The only other person in the club Rin has a relationship with is Ayako Mitsuduri, so it could be her. Nevertheless, Rin cares very deeply for Sakura despite the deal between their families, and I think I remember her mentioning she could only watch Sakura from afar and would have rather protected her if she had the chance.