I couldn't see a sinewave over 4.9 MHz crystal on my ads1232 board as well, when I put an oscilloscope probe on the input. Same time I see a small one , 100-200 mV signal if a probe on the output side. Data sheet for ads123x family adc tells that oscillator circuitry is very low power, though I 'd think that probe introduces a small capacitance, 20-30 pF, and it's enough to freeze an oscillator. Could be same things for ads1256