A. Although a father in law or mother in law is considered Mahram for the daughter-in-law or son-in-law in terms of Shari'ah, but if the age gap between the two is not big then they may not travel together on long journeys.
If the father-in-law or mother-in-law is very old or very sickly, then this ruling will be relaxed. Nowadays ‘very old’ will be an age of 70 and above.
And Allah Ta'aala knows best.
Mufti Siraj Desai