The funny thing about using branch theory instead of predestination is Skynet's and John Connor's original decisions to send fighters back to 1984 are exposed as irrelevant to their own survival. Their past as they know it is unchangeable because the time machine sends people to alternate universes. The proof is the passage of time between the Terminator going through and Reese going through. It was at least seconds, probably minutes and maybe hours, based on the amount of knowledge and preparation Kyle reveals (learning nothing dead will go, volunteering for the mission and realizing it's "a chance to meet the legend", and being told the time machine would be blown up after he went though). How did that transpire if an unchallenged Terminator destroyed their past?
Not necessarily.
Once you get into Quantum Mechanics things go way differently than intuition tells you so. Most importantly; multiple possibilities can exist at the same time, until a certain probability gets satisfied (most often when an observer looks at things) and the whole thing collapses just into one option.
Let's repeat Shrodingers cat as a simple explanation:
We get a cat. We put him in a box. We put a radioactive rock in with the cat and close the box. Now, the radioactive collapse of the rock is a probability function. How much radioactivity the cat absorbs is thus a function of that function. And whether not the cat is dead, in turn is a function of that function.
In short; the chance that the cat is alive or dead, is dependent on one function of chance that contains this all.
We wait; time passes. And somewhere along the way, the chance the cat is alive is about equal to the cat being dead. At this point, according to quantum mechanics, the cat is indeed BOTH alive, AND dead at the same time. Both possibilities exist simultaneously in a state of flux. It isn't until someone opens the box that the probability function collapses and either one or the other becomes a reality.
If you would apply this to multiple realities/timelines, if both options have a high enough possibility of occurring, both would would exist in their own timeline. However, if the possibility of one option is too low, it would not become its own timeline, it would simply disappear. (Of course, wait long enough, and the chance the cat's still alive would become so low the probability function would collapse on its own without anyone having to look at it.)
Time travel scenario would essentially be the same:
1. Terminator goes back in time. There is nobody there to stop him, he kills Sarah Connor, Skynet wins.
2. However, this has a certain possiblity of occurring; and the future timeline where John Connor defeats Skynet and sends Kyle Reese after the Terminator also has a possibility of occurring, and thus both options exist simultaneously at the same time.
3. Giving John Connor enough time to send Kyle Reese through the time machine. Which has its own probability of occurring, and so the options, "Terminator alone kills Sarah" and "Kyle Reese goes back as well" exist for a while at the same time, until something makes the function collapse; resulting either in two time lines, one for either option; or one of the options has too low a probability of occurring, and simply entirely disappears.