From Writers Helping Writers:
Description: an in-law relationship occurs when a marriage or like-union occurs, bringing two families together. The partners in the relationship join the family of their other half and a bond of respect, tolerance, and (hopefully) love comes about. But while the partners choose one another, their family members “come with the package” so to speak, meaning personality or ideological clashes can often cause friction.
Relationship Dynamics:
Below are a wide range of dynamics that may accompany this relationship. Use the ideas that suit your story and work best for your characters to bring about and/or resolve the necessary conflict.
Showing genuine interest in the other’s passions, likes, beliefs, etc.
Engaging in polite conversation
Complimenting the other (on house improvements, a garden, a choice of car, etc.)
Asking about the other’s family members, job, vacations, activities
Pitching in to help when asked (childcare, helping with a move or repair)
Avoiding contentious topics to keep the peace
Offering advice, encouragement, and praise
Asking the other for their opinion or to weigh in with experience
Offering help without expectation or strings
Sharing stories about the loved one in common
Gentle information-gathering about possible changes, or areas of concern
Telling jokes or sharing funny stories
Discussing current events, politics, popular movies, books, or pop culture
Celebrating birthdays, anniversaries, graduations, and other family events together
Sharing meals or enjoying an outing together
Talking about kids (if there are any)
Prying into the other’s business
Offering unsolicited or unwelcome advice
Being judged by the in laws and feeling that one doesn’t measure up
Suspecting the other is holding back information (or lying) due to a grudge
Believing the other is trying to drive a wedge between the character and the loved one in common (a husband and wife, a mother and daughter, etc.)
Guilt trips: You never come to visit, Sarah’s other grandparents always get her for Christmas and we never do; Why do you always stay at Bill’s house and never ours when you come to town; If you loved me, you’d invite me along on the trip, etc.
Reminding the other of their mistakes or bringing up a past embarrassment
Snide remarks, haughtiness, talking down to the other, arguments
Pushing or shaming the other to adopt beliefs about religion, politics, or ideology
Forcing other relatives to take sides
Asking for something that’s inappropriate (money, to lie for them, etc.)
Going behind the other’s back and then lying about it
Interfering with how the character raises their kids
Thinking the other’s rules are stupid and so refusing to respect them
Making the other feel small (only begrudgingly offering aid or financial support, etc.)
Making demands and ultimatums: If you want to see your grandchildren ever again…
Ignoring the other’s boundaries
Voicing disappointments to make the other feel bad
Sharing gossip about the other to purposely lower their esteem and cause rifts
Conflicting Desires that Can Impair the Relationship
A parent who doesn’t like their child’s spouse seeding discord in hopes they break up
Believing the other is a threat, which leads to constant friction
Control issues (over how children are raised, how the other lives, choices that affect family members in common)
The in laws wanting to have a say in everything and the character wishing for autonomy
Disagreements over where to settle down (in laws wanting the couple close when the couple doesn’t share this desire)
Clashing Personality Trait Combinations: Controlling and Independent, judgmental and oversensitive, stingy and generous, proper and rebellious, inflexible and spontaneous, nosy and private, gullible and intelligent
Link to the rest at Writers Helping Writers