How do you get a dog to fall out of love?

My teenage sister owns a three-year-old maltese mix called Amy. During her school hours, I take care of this dog. I do about half of the daily walks, play time, and training.

About two months ago, Amy decided to appoint me her sole caregiver. She used to follow my sister everywhere. Now, while she clearly loves her, I’m the one who gets her to squeak in excitement and if both of us call her, she will run to me. This is hard for my sister, who loved being number one.

I suspect Amy’s change may stem from me doing clicker training with her, which she loves. I’ve also made even more of an effort to be gentle with her recently (she’s a very timid dog).

Now, I suppose I know the answer to my question – it looks like I may either have to do less with Amy, or start scaring her. I won’t do the latter, of course, but the former is not a real solution either…I can’t leave all the work to my sister.

Fluther has surprised me with solutions I hadn’t even thought about before – hence this question. Any ideas?