Weddingbee boards

My parents are happily married, however, such is not the case with my fiance's parents.  His parents have been separated for 19 years & do not get along.  My fiance's mother & stepfather have been together for 17 years & my fiance actually calls his stepfather "Dad" & calls his father by his first name.  My fiance's father had remarried, however, recently divorced his second wife & has since started dating someone new.  To make matters even more complicated, my fiance & his father were actually not on speaking terms for several years until the passing of his grandfather this spring, and his father is also estranged from all of his other children (my fiance's younger siblings). 

Now, these are my problems:  Where should we sit my fiance's father & his new girlfriend at the ceremony & at our reception?  We know that they will not be welcomed by any of my fiance's other family members.  And besides his siblings, we are actually not even expecting any other family members from his father's side to attend our wedding so we can't even put them with grandparents or cousins or anything like that.  Also, where should we put my fiance's former stepmother & his younger siblings at our ceremony & reception?  Neither his father & his new girlfriend, nor his former stepmother & his younger siblings, will be related to anyone there except my fiance & we will clearly not be putting his father & his girlfriend & his ex-wife & their kids all in the same row at the church or at the same table at the reception.  And I clearly don't want any of them seated with my fiance's mother & stepfather either since his mother & father don't get along either.  And we were planning on putting my parents with my grandparents & my aunt & uncle, so we can't put them with my parents either.  Help!

@FloretteLiz:  As explained, in my post, we are not expecting any other family on my fiance's side as they live on the other side of the country & will not be traveling here for our big day.  There will be only his mom & stepdad, his father & his new girlfriend, his former stepmother & younger siblings (who are all still either teenagers or children), & his one older sister & her husband (who we are planning on seating with my fiance's mom & stepfather).  So to answer your question, no, there is no other family to put with them.  My fiance's guest list is filled with friends & coworkers, not family.  This is why I am having such a difficult time.  I have read all the traditional advice about where to seat divorced parents.  It does not apply to this situation as we have no other family to mix in with them to keep them separated while still having full rows at the church or full tables at the reception.