The simple answer to this one is that I have six siblings, so family relationships are part of me. I’m also number six in the queue, so I was born into a situation where I had many sibling relationships to observe as I grew up. Then too, I love my brothers and sisters, and consider each one a friend, so that’s also an organic part of what I write.
There are other reasons to build siblings into a story, though.
First, the hero and heroine need what are called reflection characters. These are the side kicks, mentors, sometimes the devil’s advocates, henchman/women, and other secondary roles who enrich the protagonists’ worlds and make their stories complicated Siblings fit well into these roles, and using siblings helps me get to know characters who may soon have books of their own.
Second, a lot of what troubles the hero or heroine probably comes from their past, and few people will know that past as well as siblings do. Think of Lady Eve, and how her sisters alone knew how hard she’d fought to recover from her fall. Only Westhaven’s brothers knew how much pressure he was under to marry. Ian MacGregor held Asher’s secrets, and was the one encouraging Asher to trust their siblings with those confidences.
Third, because the hero and heroine are carrying around old wounds, they often have to sort things out with family members before they can rise to the challenge presented by the romance. Ethan and Nicholas had to air old laundry before Ethan could move forward, same with Sara (Beckman) and Polonaise (Gabriel). The family of origin stuff will hold us back until we deal with it, and then our family ties can propel us forward into a happily ever after.
Fourth, part of what every romantic protagonist has to learn is that they are lovable, and worth being loved. It’s not enough to learn this only as it relates to their partner. That’s a life lesson, and means accepting the love and support of siblings as well.
Tons of reasons to build siblings into a romance!