We're in such a weird time for events and gatherings and such. I realize it's too late for this, but right now I think it's important to think through risk tolerance before planning any gatherings. If the guest of honor (or other must-be-there-guests) will be uncomfortable at an indoor gathering w/ unvaccinated people (which is totally understandable), then they should either plan an outdoor gathering, or be clear about vaccination requirements when creating the guest list and inviting people (I'm sure some will disagree, but I don't think it's horrible to only invite vaccinated people). It would be rude and a disappointment to an attendee to be invited, RSVP and plan to come, and then be disinvited after the fact.
Based on where you're at now -
*my personal risk tolerance would be okay with the party as is, as long as it was an adult-only party/my kids weren't attending (they are too young to be vaccinated) -- I'm pretty confident in the protection I get from the vaccine and the protection it provides against transmission, so the major burden of risk is on the unvaccinated family, and I'm at the point I feel that every gets to make their own choices about risk
*I think it's totally legit/not rude to do a "we're following CDC guidelines for this event" and a reminder that unvaccinated people must wear masks -- but I think the likelyhood of it being done and being done well is low and you risk an uncomfortable situation at the event itself that singles out the unvaccinated family. First, b/c they'd be the only people wearing masks, and then second, because what happens if they wear them, but incorrectly? If you don't have someone willing to constantly be saying "please put your mask back on, please put your mask over your nose" then I don't really get the point of asking them to wear them -- and I think you risk REALLY alienating this family if you decide to have someone police them (and only them!) all night
*Ideally, the gathering could be outside, at which point this all becomes a non-issue to me and I'd have no issues, even with my unvaccinated children attending.
*Another less alienating option is just to ask that EVERYONE wear masks -- "the guest of honor is recently recovering from surgery, so in an abundance of caution, we ask that all attendees wear masks when not eating" -- that doesn't alienate anyone, and it's also way easier to enforce it if you're doing it to everyone, not just one single family. This also fits w/ my own personal philosophy that I'm totally okay to ask others to wear masks around me -- but only if I'm willing to wear one too.
I also agree with PP's comment that you should understand the guest of honor's opinions on this -- my 76 yo aunt is kind of like PP's mom -- she's careful, but at this point she'd prefer to go back to living "normally" even if it's a little less safe for her -- she vaccinated, but happily sees my unvaccinated DDs now, b/c missing another year with them isn't worth it to her. If the guest of honor feels like this too, then move on as planned.