I am a stuffy old goat, I know, but I suppress a polite shudder any time I receive an invitation to a private event that looks like it was extended by an entire social committee. The actual rules of traditional etiquette are flouted all the time, so don’t hesitate to carry on doing so where they don’t apply, but I am going to tell you what they are (so that you can flout them on purpose instead of accidentally), and try to explain why they are (so that you can decide advisedly which ones to flout).
First: polite people keep their financial arrangements private. Invitations do not advertise who is paying for anything, and paying is NOT the same thing as hosting. “Hosting” means taking full personal responsibility for the safety and comfort of the guests under your roof — or at least, the roof you have rented for the occasion. The hostess is the person who makes the final decision on the guest list, the menu, the entertainment, and the arrangements.
Second: at a wedding, the hostess should be the bride’s near relative — or, nowadays, the bride herself. As a result, the wedding is supposed to be in a style, however modest, that the bride’s family can afford. This is because the bride is seen to be leaving her family to become part of her husband’s family. This is her family’s last chance to act on her behalf. If she relies on her husband’s family even before the wedding, it makes her look like she is ashamed of where she comes from and is marrying for wealth and status.
Third: any social event should be hosted by one hostess. If she is married, her husband may optionally co-host. This is because hosting is a responsibility — quite a big one — and for any role of great responsibility there needs to be one place where the buck stops.
Fourth: the purpose of an invitation is NOT to “honor” special VIPs, but to inform the guests WHO is inviting WHOM, to WHAT, WHEN, and WHERE. So, in this case (assuming you have decided to flout the second rule above; which is, it must be admitted, arguably rather sexist) WHO is “Mr and Mrs FIL”. The totally hide-bound utter-etiquette-stickler traditional wording would then be:
Mr and Mrs John Fil*request the pleasure of the company of
leave a space* to write in guests names
at the wedding of
Miss* Smho Nig
*Mrs Step Father
*Mr Birth Dad
to their son
on fourteen May at two o’clockat Saint Thomas United Church
*The host and hostess could just as well be referred to as “Mr John Fil and Ms Jane Fil” if your future mother-in-law prefers to see her given name in print, or even “Mr John Fil and Ms Jane Mil” if she has kept her own name. She can even use the title “Mrs” or “Miss” with her own name if she prefers and we sticklers will barely raise an eyebrow.
*If you plan to use inner envelopes, you don’t need the write-in line on the invitation, because you can specify WHOM you are inviting on the envelope instead. In that case, just use “request the pleasure of your company at the wedding of”
*You get the title “Miss” (or “Ms” if you are more modern) because you are not yet related to the hostess;
*Your parents sneak onto the invitation as part of the WHAT section where they help your more distant (forgetful) relatives remember who exactly you are. Your step-father gets on there obliquely by means of your mother’s using his name. If that isn’t good enough or if your mother uses her own name, you could change this part to “Mr Step Father and Ms Birth Mother” even though it isn’t technically accurate and will put your father’s nose out of joint.
*Each parent gets his or her own line, because they are not married to each other and you never “write up” unmarried persons together on social correspondence. Technically, that means that if you end up hosting this yourself, it should be just you and not “you and future husband”. It also means that any non-living-together couples you invite as couples should each get a separate invitation instead of “Mr John Myguest and Ms Anne Guest-of-guest”
*Your darling fiance loses the title “Mr” because parents do not refer to their own offspring by titles.
Fifth: in safe social situations, ladies come first. This does NOT apply on the outside of envelopes, no matter how many people wrongly tell you otherwise; but it does apply in safe social or ceremonial circumstances. So at the church, your mother sits in the first row on audience left.
Sixth: You do not sit divorced couples together (ever) or break up married couples at the ceremony (you do, at dinner, though). This is because divorce is awkward, especially in the context of a wedding, and you do want to show respect for the institution of marriage, which includes the real and persisting marriage of your mother and step-father. So your step-father sits with his wife, and your father sits in the second row. What you can do, though, is ask your mother and step father to sit somewhat inside the pew farther over to the left, so that there is no-one sitting in front of your father in the second pew and he sits more centrally, right next to the aisle. If he has no wife, let your grandma sit with him to balance things.