What the hell was I thinking two years ago?
In Fate route of the visual novel, she helped Saber with some foreplay in order to arouse Shirou. Shirou, the hypocritic twit, would not hurt others to supply mana to Saber. However, he was open to supplying Saber with mana via sex. During the freaking intercourse between Saber and Shirou, Rin couldn't help herself but masturbate in the damn corner.
In UBW, she is straight as an arrow. Instead of Saber, Shirou needed mana. Who was up to the job? Rin. Saber was outside looking at the moon.
I haven't play Heaven Feel, so I'm not sure if Rin and Sakura are into incest.