I mean this:
"Landing. Aircraft, while on final approach to land or while landing, have the right-of-way over other aircraft in flight or operating on the surface, except that they shall not take advantage of this rule to force an aircraft off the runway surface which has already landed and is attempting to make way for an aircraft on final approach. When two or more aircraft are approaching an airport for the purpose of landing, the aircraft at the lower altitude has the right-of-way, but it shall not take advantage of this rule to cut in front of another which is on final approach to land or to overtake that aircraft." -14 CFR 91.113(g)
If you're about to turn base, and you see that doing so would require a plane on final to alter their flight path, then it's your responsibility to avoid the conflict. The easiest way would be to extend your downwind far enough so that you can turn final a safe distance behind the plane that is already on final.