At that point, sh, she might just make it so the jewel maid has the same effect so she or someone else constantly feels what it should be feeling. On the other hand, even though they're smooth, her gems are still cold, hard rocks, so, maybe not so good for the sexy-times. The slime does have precedent, though, with Lord El-Melloi II and his mercury maid. Plus, the more fluid shape would be easier to hide (in a jar, for example), and more malleable for the sexy-times. Just change the shape of the slime to be whatever she wants at that particular moment.
Now, in the spirit of the season...
It's Christmastime in Fuyuki, and all Shirou wants is for his two favorite people in the world, his dearest Onee-san and beloved Master, to get along for a little while. He just wants to spend the holiday with the two people most important to him, without them fighting like they always wind up doing. So he asks them, and with a little work to convince them (they would have said yes anyway, but think it's important that he not just think they'd tolerate each other just because he asked), the estranged sisters agree to spend the few days around Christmas at the Emiya house, trying to get along and doing the holiday thing.
Of course, they also see it a little bit as a competition, trying to establish superiority in the time they're both there. Dressing as sexy Santa or elf to try and get Shirou to pay more attention to them, mistletoe things. Of course, there's the inevitable gifts; aside from whatever else they might have gotten, Shirou also gets other more special gifts (being allowed to have either vaginal or anal sex with Rin after she'd been restraining him for a long time, something for Sakura. I'd imagine she'd already done basically everything she could think of with Shirou by this point). Rin includes a gag gift of a pair of her stockings, for Shirou to "use" when he's feeling the "urges".
Then Shirou gets upset with them for still competing when all he wanted was for them to get along, and he runs off. By the time he comes back, Rin and Sakura have realized they screwed up, feel guilty, and decide they really will spend the rest of the time they're there doing just what Shirou asked for: getting along and sharing in their domination of him. Double footjob, one riding his face while the other rides his dick, and with all the double femdom sex, there's peace in the house of Emiya for a while.