I mean you already have a beginning. If you have a ending in your head for this you can play around that, if not you can write in the middle whatever you want. no one will complain if make sense. From example: MC gets cancer but hide it from MC sister and when she win whatever she is trying to achieve, MC tells her or she found out. Another example: MC starts to be too aggressive as a coach and MC sister ends up hating him.
i know is easy to write from the outside, but if you have in your head where are you now and where you want to go from there you will have more ''freedom'' to make the game reach its goal/ending
I was formerly a screenplay writer, most often based on existing properties (novels, short stories, novellas).
I have an IMDB page.
FOR ME, wiriting something appropriate for TV shows or movies is easy.
I have also attempted writing novels of my own. I have finished a handful. None have yet been published.
Some stories just flow out of me until a certain point. In general I let the characters take me where they want to go, and where it makes sense for the story to go. In general something like 2500 usable words can flow out of me a day if I'm going to a day job and doing real life in general. That's about five pages after edits and everything else that needs doing to make those words "fit", and to ensure that I am satisfied with them.
HOWEVER there are generally events in life that put a damper on those numbers. An extra stressful day at a day job Ihappen to be holding? I'll be lucky to get 500 usable words, and some days are zero. Some days will become negative numbers due to revisions. Maybe I though of something that changes the tone of the story and pages, chapters, entire sections need rewriting.
Writer's block is real, though in my case it depends much more on the setting, and my state of mind, ongoing stressors, and so on. I was some hundred and fifty usable pages in to a story and had to move residence in the middle of it. I moved some place where writing... wasn't really a thing I could achieve. I forced another dozen pages or so of the story, but that was it. The flow was no longer in me.
While moving house, there wasn't time to write, nor did I have access to my (then) desktop computer to do writing. The moving itself was quite stressful. Getting things allocated into storage vs living space, making sure I had enough money to get by day to day, pay for a new, more expensive place, in a city I did not want to be, trying to get along with housemates suddenly, all of that contributed to a state of mind that made writing untenable.
Most people are capable of writing. Think about essays in subjects you had no interest in for school, were you able to write them? Sure.
It takes talent and skill to write things that interest others. Things that flow, that maintain consistency, and are entertaining.
Sure you have the basic ideas. Can you put them into words in a way that will interest others? More importantly can you do so in way that would satisfy YOU?
I have had the opportunity to take others' words and turn them into something practical for another medium. I know the difficulty involved in doing so, and moreso I know the difficulties in making one's own creation that is enterrtaining and interesting enough for others to read, but even more important, something I like, something with which I can be satisfied.
No, it's alright to write a bit about cooking etc., nevermind. I like cooking myself and have a few favorite recipes which I would gladly share if I was writing something similar. Honestly with that comment I was mostly just trying to poke your imagination, unassumingly reminding you about the Sistersitting and maybe to provoke some thinking about MC, which might rekindle some inspiration and a wish to write another scene or something. So I'm guilty I had ulterior motives 
