Calculating the maximum basis voltage of an transistor
For silicon bipolar transistors you may assume that Uebrest is approximately 0.7 volts. From the facts you have presented it is not possible to calculate this. If you want to analyse your circuit by logic reasoning, assuming 0.7 volts for emitter/base is a good help. However, if your only goal is the calculation of Uebrest itself... sorry, can't help.

I've got a sneaking suspicion that you are not able to express your real problem, at least it seems that you don't understand what I am trying to say. A practical approach to transistor usage is that the arrow on the symbol is the emitter, and current only flows in the direction of the arrow between collector and emitter. The current will also only flow if the emitter-base voltage is above 0.7 volts (rule of thumb), the lowest potensial being at the arrows end. Base voltage should also be less than collector voltage for PNP (arrow pointing out), or base voltage greater than Collector voltage for NPN (arrow pointing in).

Generally you use collector for voltage amplification (inverted output) and emitter for current amplification. Your circuit resembles an amplifier, but the transistors are not set up correctly. It seems to me that you have put the NPN transistors in "up-side-down". That probably why you are having trouble.