Guest list etiquette

Okay ladies, I need some input.

Back story: I go to a small, tightly knit church.  We throw a shower for every bride-to-be and every pregnant mother, whether its her first baby or sixth.  It has been conceded on this board before that church showers are "different" and that its fine when churches throw modest showers for every baby.  

The church has a fund of $75 for every shower, no matter who or what its for.  Excesses are covered by people volunteering to bring things or out of pocket by me and the other hostess.

I am on the planning committee (which really is just me and one or two other ladies).  I don't make all the rules, per se, but I have some sway in what happens.

Question:  I have a dilemma about who to invite to these showers.  Here is a particular example:  We had a wedding shower for my SIL (husband's sister) last fall.  She is young and grew up in the church and most of her closest people are members of the church.  Most of her extended family lives out of state.  She had one bridal shower, to which all the church ladies, and her out of state family was invited (knowing the OOT family would mostly not come).  Between the church people and her family, that covered 95% of all the people who she would have invited to a shower.  But that leaves those 5% of random work friends, parent's friends, whoever.  Really not enough people to throw another shower for.  One of those 5% is my mom.  She wasn't invited to the shower because she's not really related nor a member of our church, but she's known my SIL for many years and really she should have been invited to a shower of some sort.  

Now I'm stuck.  Some of the women just have a second shower, but those are usually the women who have lots of in town family or work friends, etc.  What to do about the women who maybe have 10 or less random invites?  The church shower is funded by the church and is sometimes a quasi-potluck by the members.  It really is intended to be a church event to which a few other people are invited, not just a regular shower where the guest list is very mixed.  But I really hate to think of the women who might expect to be included, and are disappointed.  (This is really on my mind now, because I didn't think to invite my mother to my SIL's shower, and now my other SIL is getting married and she vaguely hinted that she was hurt she wasn't invited to 1st SIL's shower).  Should I just invite the random invites even if the guest list is then 35 church members and 9 random people?