Brothers & sisters, sons & mothers, fathers & daughters,....always worked just fine for me. When I was breeding and raising feeder mice. If a female gives birth, to a live mouse, that a snake will eat,....
what's the problem?
It will take many generations of serious in-breeding and time for problems to occur.
A lot of the hangups and taboos that we have, in this regard, are dreamt up by humans. A species with a proclivity towards dreaming up things.
Animals, left to themselves, need much less Psychiatry and much less Sunday School than some humans seem to have evolved to "need".
EDIT : Note, to clarify, I am talking about the pairings of mice/rodents here. It occurs to me that this reads not-quite-right without the clarification.