I wanted the kittens to return to the back yard, but didn't want to accompany them there via the side yard, as I was sure Portia would follow, but I eventually got both kittens to enter the house through the garage door.
They didn't seem surprised to then find themselves in the den, a room they've explored before. I finally got them to leave the house through the back door, but only after both of them had made side excursions through the kitchen and into the living room.
After I got them outside, I went back through the garage to see what Portia was up to. She had watched the kittens intently the entire time they were out there, and had glared at me when I let them indoors, but now she was gone. I looked along the side yard, but she wasn't there. I hope she has decided not to go hunting for the kittens. It's been about ten minutes since then, and I haven't heard any fights break out, so that event might have been postponed for a while.
The third kitten, the gray tabby, was nowhere to be seen during this entire near-drama. She's the least adventurous of the litter, and I hope she has the good sense to avoid Portia, even if the other two don't. I also hope that the other two have had their fill of adventures for the night, so I can stop expecting to hear a cat fight at any moment. I have things to do, and cant be cat-sitting all night.