DL requires 70K miles for a round-trip (no one-ways), so you will end up needing to purchase 1 round-trip plus the two one-way tickets.
UA is 32,500 miles one-way so unfortunately you only have enough miles for a one-way ticket, meaning you will need to puchase 2 round-trip tickets + the remaining one-way ticket. AA seems to be your best bet, at 35K pp each way.

AA is showing a lot of economy MileSAAver seats through December 14, at 35K per person, one-way. I'd book the two award one-way seats PEK-CVG for your fiancee and her daughter, and then buy two round-trip seats, including the same PEK-CVG return leg so you are all traveling on the same flight for the trip home (which sounds important to you). The two revenue seats seem to be roughly $900-1200, depending of course on your exact dates of travel. One-way tickets are practically the same price in $$, but only half the miles, so this seems to give you a fair amount of value. Total out-of-pocket cost would be roughly $2000 and 70K miles, leaving you 43K miles - enough for your fiance's other daughter's flight later this year. Additionally, you'll acquire more miles on the two paid tickets for another future trip.

All this booking can be done online, in two separate (though possibly even simultaneous) transactions. Afterwards, any agent should be able to link your reservations to show you are traveling together on the return leg. Book the award tickets first, to ensure availability, and then book the revenue tickets. They are unlikely to disappear in the few moments between transactions. Alternatively, you could book the award tickets over the phone - an agent can put them on hold for a few days, so those seats are confirmed while you purchase your paid tickets (online or via a separate phone reservation). Then complete your award transaction with no risk.