Is it unsafe to cross ones legs after partial hip replacement?

Yes it is good not : Do that it may dislocate (get out of place).

It can be: During the healing process of any hip replacement procedure you try to avoid provocative positions of your hip joint to decrease the stretching if the repaired tissues opened up to get into the hip joint to install your new parts. Crossing legs stretches the repairs and can cause a tearing of the sutures or stretching of the tissues that you want to scar tightly so your hip does not dislocate.