From Emily Post's Wedding Etiquette, 4th Edition, helped along by Peggy Post.
Which I bought for just these stepfamily sorts of reasons.

First wedding purchase I made!
Traditional wording...For the bride and groom who cherish long-standing traditions, conventional wording and spelling will govern their invitation choices.
goes on to...
When divorced parents give the wedding together
In the event that relations between the bride's divorced parents (one or both of whome may have remarried) are so friendly that they share the wedding expenses and act as co-hosts, both sets of names should appear on the invitation. The bride's mother's name appears first:
Mr and Mrs Shelby Goldring
and
Mr Michael Levy
request the ........
If, however, the bride's parents are not sharing expenses, yet the bride wishes both aprents' names to appear, a different situation exists. If the bride's mother is not contributing to the cost of the wedding, the bride's father's name appears first on the invitation, and he and his wife host the reception. The bride's mother is then included only as an honored guest at the reception.
I got this book b/c I was inviting long-time friends of my mom, who hate my dad, but my dad was paying for my wedding. My mom had died before I eve met my now-husband. I wanted to do things "properly" so I had something "behind me" when my mom's friends confronted me, if they would do so. I'm an informal person normally, but I knew this was going to bring weird things up, and I didn't want to be "alone".
And while they didn't confront me, some didn't come, because I had the gall to put my dad and stepmom as the hosts, but since they WERE the hosts, I still feel I did the right thing. To have put that hubby and I were inviting people to a wedding we planned by didn't pay for...just didn't feel right to us, though MANY people do that nowadays so as to not hurt feelings. To me, if you're not paying for it, you shouldn't have your feelings hurt unless you talk to me about it well beforehand (my stepdad, it turns out, felt that his generous gift when we were engaged had paid for space on the invite, but he never TOLD me that, LOL).
so anyway, since you're co-hosting, per the Posts (unless you want to just let the kids "send" the invites without your names on them), you as the mom would go first, then and the dad's name, and the stepmom could go there if everyone wants her there. For me my stepmom being on the invite was a given, as she's been my stepmom since I was around 7, and she was the one working and therefore paying for it, LOL. I did have it written as "Mr and Mrs B invite you to the wedding of his daughter", because otherwise it would have felt like my stepmom was my mom, and even though she IS my only mom now, she wasn't always, so I couldn't include her in "being" my parent on the invite.