And the nuns make it on time!

Here is the solution:

Sister Speedy and Sister not quite so speedy go across (2 minutes total)
Sister Speedy returns (3 minutes total)
Sister at least I'm not the slowest and Mother Superior cross (13 minutes total)
Sister not quite so speedy crosses by herself (15 minutes total)
Sister Speedy and Sister not quite so speedy cross (17 minutes total)

Alternatively, Sister not quite so speedy can go back in the second step, and Sister Speedy crosses by herself in the fourth step with the same result.