Can I clean the brushes in alcohol for re-use?

Re-using the brush is not really recommended as you cannot guarantee how thoroughly clean the brush is, it is better and safer to use disposable brushes

I would not recommend reusing the brushes or trying to clean them.  Latisse should have been supplied with the appropriate amount of brushes for the size of bottle you purchased.  

It is not recommended to clean the brushes for reuse. Simply toss them and get a new one. The material they are made of is not designed to hold up to cleaning. They are designed for single use. "This answer has been solicited without seeing this patient and cannot
be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."