All IN-LAWS - whether they are in the form of Mother, Father, Brother, Sister, Son or Daughter, are to be treated with a great deal of caution from the very beginning.
These people from both sides - and that include practically all of us - have very pronounced and often well hidden conflicts of interests. They want to keep their son/daughter or brother/sister in the sphere of their influence. At the SAME time, and often quite genuinely, they wish them good luck and happiness.
This type of conflict is only potential; but it almost always leads to the readiness to criticize a new member of the extended family.
One of the ways to set this up is to express criticism to one's own family member behind their new partner's back. As a result, the listener is placed in a position of disloyalty to the absent partner.
If, however, the criticism is expressed in the in-law's presence, then she/he has the option to respond directly. Hopefully the exchange will be conducted in a civilized manner.
A not so positive outcome would be to say nothing to the critic, but to wait until later to attack the partner by complaining about the behaviour of her/his relative/s. This will create a defensive reaction and lead to more and more conflict. Then, a life-long ordeal, which sometimes continues throughout the life of generations, begins. Another example of conflict of interests is the situation, in which the family of origin tries "to help" their son/daughter, brother/sister, who is in a conflict with his/her partner/spouse. The family is only aware of the negative feelings their own one has against the other. It has no idea about the controversial feelings of love/hate, "fors" and "againsts" the participant is experiencing. The family of origin is only aware of the anger expressed by the family member. That anger is only a part of what s/he is really feeling. Wise parents tell their adult child who complains to them about his/her couple difficulties: "marriage is hard work. Things happen - try to work it out." Of course, this is an appropriate advice when there is no abuse involved, only a fight or a quarrel. Trying to "help" is sometimes similar to when a waiter is carrying a well balanced but heavy tray and a do-gooder wants to help by putting his empty glass on this tray. The tray may then overturn - often right on top of the do-gooder. When people fight, their judgment is not the best, so to offer them comments such as: "how can you tolerate it?", "don't you see what s/he is doing?", etc. is equivalent to "insider trading".