The other egg has pipped and I could see the beak and so I gently used tweezers to remove the shell around the air sac. The membrane was dry so I moistened it with coconut oil. The chick is making swallowing motions and the membrane shows veins still.
Good thinking so far. That sounds like the chick is not really ready to finish hatching yet, probably still absorbing the yolk.
What do I do to help it hatch now? I don’t have an incubator and Yinny is busy with the other chick.
I have another broody sitting and she is at about 8 days. Should I put the egg under her? Should I bring the egg inside and keep moistening it until it hatches? Please help!
Gosh, that's a hard one. There are several things you could do, but I don't know which would actually be the best.
As a short-term emergency measure, I would probably try putting the egg under the other broody, but keep a close watch on her. Some broodies will accept a chick who hatches that soon after they begin sitting, and some will attack the chick. I would hope she can let it stay under her for long enough to absorb the yolk. Or at least long enough for it to stay warm while you figure out something else.
If that does not work, or you don't want to risk putting it under the other broody, taking the egg inside would be the next logical thing to try. Some people make emergency incubators from a box with a light bulb for heat, or a jar of hot water for heat. No matter what heat source you use, you will need to use a thermometer to keep the temperature right, and you will need something to keep humidity up (a wet sponge or wet cloth near the egg is a good first choice for that.) The internet tells me you want a temperature between 37 and 38 celsius for chicken eggs to incubate or hatch. (I am used to thinking in Fahrenheit, but I notice that you are in Australia. So I checked for the right celsius number.)
After dark tonight, you might be able to put the egg back under the mother (Yinn) for the night. Broody hens usually stay in one place while it is dark.
Once the chick does hatch, I think there is a good chance that you can have Yinn raise both chicks, unless she decides to reject the late one. (Some hens are fussier than others about taking in chicks, so you will have to just try it and watch how it goes.)