My understanding is that bats aren't particularly closely related to shrews at all, though I've seen it said that they have a "shrew-like ancestor." It helps not at all that there are about four different families of mammals called "shrews" - tree shrews are related to primates, elephant shrews to aardvarks, plain ol' shrews to moles....and that the fossil record of bats is pretty sparse, too.
A lot of work seems to be in progress on the relationships among mammals in general:
Science,
291, 1786 and
294, 2348, both from 2001, might be places to start.
And there are a variety of critters that "fly" to some degree - even a small snake from SE Asia that has a flattish body and squirms through the air to be a better airfoil.