If you are already using Calibre then I would just look up Regex Functions in the
Calibre User's Guide. There is an example in there for Title Case, but that could just be slightly modified for Sentence case.
Sorry... I haven't used Calibre's Editor since Sigil came alive again, so I'm not as proficient with it. If you have issues you may find some smart Calibre people on their forum...although there are a few that poke their head's in here occasionally!