Immediately after sending that, I realized you are primarily an action focused writing blog. You don’t have to answer it!!
The answer to your question is simple in concept but complex in execution. It may not really feel like an answer at all.
Writing a relationship between half-siblings is like writing any other kind of family dynamic. In this sense, it’s freeing because any answer you come up with is going to be right. However, the success of the relationship is dependent on establishing the dynamic, family history, the relationships between their parents, and how that fallout affected their children.
Like full blooded siblings, half-siblings can run the gamut between best friends, my brother, space takers, “eh”, just there, virtual strangers, friendly rivals, bitter rivals who still love each other, and mortal enemies. It’s entirely dependent on their relationship dynamic, their personalities, their relationships with their parents, and their parents relationships with each other.
So, don’t get caught up in trying to build their relationship based on what “should be” in accordance with society. As you establish these characters, let your own imagination lead you to what their relationship is.
When you set out to create siblings, you can start from two directions.
1) The Outside Looking In.
On an Outside Looking In approach, you begin at the beginning and start by establishing these characters’ parents, their history with each other, and lay out the groundwork for their background before they ever entered the picture.
“Rob married Sally and begat John. When John was five, his father divorced his mother and married his secretary Margaret who was pregnant with his sister Jane. Sally received full custody of John, he sees his father and his father’s new family only on weekends.”
From this bare bones outline, we don’t get much of a sense for how John or Jane feel about the situation. We do get a good sense for the interrelations between their parents, the underlying betrayal and sense of loss that might influence any relationship between the siblings.
Once you have a set up, it’s easier to start adding in characters because events are often easier to contextualize than feelings. When you’re on the outside looking in, the whole story seems simple, and the more you dig in then the more complex it gets.
2) The Inside Looking Out.
The Inside Looking Out approach begins with the siblings themselves, or even just a single sibling. Their character, their likes, their dislikes, how they feel about their sibling (if they feel anything at all), using the emotional clues we can begin to work our way outward to establishing what caused the character to feel this way.
“John is sixteen, nearly six feet. He likes trains, aquariums, mathematics, hates reading, and is very fond of his mother. He styles himself as the man of the family and keeps up a part-time job on top of his schoolwork to help his mother with expenses. He doesn’t look forward to the weekends because his dad has been pushing for him to look after his half-sister Jane, who will be joining his school as a freshman this year.”
Looking at this scenario, we get less clues about what happened in the past but we know more about John. We also know that John has a bone of contention with Jane over the amount of time their father spends with each of them. John’s mother never remarried and he’s been busy trying to be the “man of the house” but is always constantly reminded that his sister is getting the father that he never got to have.
On the Inside Looking Out approach, we start to see how the father is the central issue in the lives of these half-siblings, the only connection the two share, and a source of resentment. We may not have all the contextual clues of the greater scenario, but we know how John feels. When asking why, those feelings can be the sources which lead us backward to the whole of his background leading up to the story’s beginning.
All relationships are relationships and all relationships are contextual. There is no real “right” way to write human dynamics.
Learn to listen to your characters and what they’re telling you about their relationships with each other.
Don’t ignore the bad.
Human relationships can be fraught with drama.
Even the best of them can be be petty.
No one is perfect all the time.
Try not to take sides, even if you have a favorite.
Try to figure out how these characters got here and why they are feeling what they’re feeling.
Examine your relationships in your own life and those of your surrounding friends and family.
Practice asking, “why do I feel this way? What caused me to feel this way?”
Creative Fiction is an expression of humanity.
Humanity is a complex beast.
Find the points of tension in their relationship dynamic. The tension with each other, the tension with their parents, their sources of happiness, their insecurities, and everything in between.
Basically, don’t stress. Just start writing it and see what happens.