Make-believe games are important for young children. It's a primary means of learning. They imitate, as best they can understand, the activities they observe in adults and older kids. Repetition is a primary characteristic of imitation games. And of course, she sees her adored mommy and fascinating older sister use the bathroom several times a day, so it must really be important, right?
Little ones long to help cook, clean house, care for pets or babies, rake the yard, and, if you're lucky, use the toilet and/or wash hands. There is probably little distinction in your daughter's thinking between bathroom make-believe and any other imitation she might engage in.
This is a chance for you gently support her fascination; it will probably pass as soon as she either masters the "game," or discovers she's not yet able to master it.
I would just stay cheerfully matter-of-fact about it, and some other activity will eventually capture her focus. The week or so that she's given to it is far from excessive. If you reported three months, I might have a different set of concerns.