Forget about yuri for a moment. The reason Kirira is interested in Sayaka is because Sayaka was able to instinctively tell her apart from her twin. Sayaka was so interested in Kirari (the real one) that she could tell right away the Kirari she was talking to was a fake. That's why Kirara took her as personal secretary.
Likewise, I think Ririka is interested in Mary because Mary is the only one who sees Ririka as herself and not just a shadow of her sister. Note how Mary told her "think for yourself" and also had her drop the mask. All this really touches Ririka's heart.