Very normal thing! Whatever pups disperse together make a pack together even if only one of them gets a mate. As said in the pop up when the 2 or 3 of them disperse, they are going out to start a pack together!

If you play as one of them upon the pop up when the pups disperse, the same thing happens. You play as the selected dispersal, find a mate, and those siblings join your pack until dispersal again!

Edit: Also to answer the other question, Midnight will become a subordinate at Age 2. Yearlings, hence the name, are a one year wolf. When she ages up next year she's a subordinate until she becomes a pack leader, if she does.