Is congratulating someone who has returned from travel on their safe arrival part of the etiquette of Islam, and how should I deal with my mother-in-law who did not congratulate me on my safe arrival and believes that I should have called her first?

Congratulating someone returning from a journey is a good custom. The Companions used to embrace each other upon their return from journeys, and the Prophet, peace and blessings be upon him, embraced Ja'far upon his arrival. There is no harm in what people have customarily agreed upon in these matters, as long as it does not contradict Islamic law. Among the good manners is to visit the returnee and greet him. If someone falls short in this, the matter is easy and should not be a cause for estrangement. It is recommended to treat the husband's mother well, visit her, and give her gifts, for good character will weigh heavily in the scales on the Day of Judgment.