Considering a minimal number of assumptions and in the context of the timeless formalism, we derive conditional probabilities for subsequent measurements in the non-relativistic regime. The probability expressions are found unambiguously, also in puzzling cases like the Wigner's friend scenario, and they underline the relativity aspect of measurements. No paradoxical situations emerge and the roles of Wigner and the friend are completely interchangeable. In particular, Wigner can be seen as a superimposed of states from his/her friend. When only one clock system is considered, we demonstrate in addition that two-times probabilities cannot be defined.