There are numerous factors that influence how people interact with one another in intimate relationships. Birth order is an often-overlooked variable.
Only children with each other, and then oldest children with other oldest children, are the most likely to divorce.
Onlies and oldests are both more nervous and achievement-oriented. As a result, no one goes with the flow, and everyone wants his or her own way. That rarely works out.
Maybe this is because kids with more siblings are better at coping with others, or maybe parents with many kids are less stressed and love kids more.
Such grandparents might help their children have less stressful marriages and minimize their divorce risk. Maybe having more kids makes you more religious, and religious people divorce less.
I think this is due to imago theory, where each child becomes used to interacting with their sibling and unconsciously replicates this pattern in adult interpersonal relationships.
In a marriage between an oldest and a youngest, both parties may feel comfortable with their familiar sibling dynamic. Oldests are more achievement-oriented and rigid, while youngers are more pleasant and adaptable.
Researchers believe siblings who are seven or more years apart to have been reared like only children in terms of sharing, parental attention, and so on.
If you're Irish twins, your older sibling is more achievement-oriented, anxious, and inflexible, while you're rebellious, independent, and adaptable.