Where is the passion?
There are some gramatical errors, but those can be fixed with proofreading from peers and/or a grammar site like Grammarly. The plot is there, but where is the passion? Passion and creativity are what drives a story and is what motivates an author. You've introduced the characters and that's great, but I, as a reader, know nothing about them except that they're step siblings. I agree that you should start by making the paragraphs longer and more detailed.
Read the story now
Show, don't tell. I know it's hard because I even struggle with it, but hard work can go a long way. Show us the relationship between the siblings leading up to the first scene. Are they close? What are their likes and dislikes? What do they look like? What is the setting? In the first scene, where you're introducing the brother, what is he feeling? We know that he knows she isn't "the one", but why isn't she? What is the brother looking for? What makes that woman not "the one"?
The same goes for the sister. Aside from what is said in the chapter, what else is she feeling? Is she frustrated? Is she jealous? Or does the situation not bother her?
The potential is there you just need to find the passion because you already have the creativity. Take the potential, the passion, and the creativity and run with it. If writier's block is the issue, make an outline of the main points of the plot, and the scenes leading up to those main points just might fall into your hands.
I've been where you are right now. I used to have short and simple chapters with my sci-fi romance book, but I went back and expanded on it. You're on the right path, you just need a little bit of guidance. Stay strong, you got this!