This question is tricky because it involves emotional and moral choices between family members. Normally, the son will decide who to save first based on his own judgment and situation, but there is no one standard answer to this situation.
From a moral point of view, the son should save the person closest to him first, that is, if the mother-in-law is closer to the son, then the son should save the mother-in-law first. On the contrary, if the daughter-in-law is closer to the son, then the son should save the daughter-in-law first. This ensures that as many people as possible can be rescued when time is of the essence.
However, in practical situations, the son may make decisions based on his emotions and judgment. For example, if the son has a better relationship with his mother-in-law, or if he thinks his mother-in-law needs help more, then he may save his mother-in-law first. Similarly, if the son has a better relationship with his daughter-in-law, or if he thinks that his daughter-in-law needs help more, then he may save her first.
In conclusion, there is no one standard answer to this question. The choice of a son can be influenced by a variety of factors, including personal feelings, urgency of the situation, distance, and more. Whatever decision your son makes, he should make judgments as calmly and sensibly as possible.