thinking about canon era enjolsette siblings....
hear me out here; them being siblings on their dad's side and being tholomyes' kids. tholomyes, being the pos he was, was actually married with a child when he pursued fantine and left her when cosette was born to return to his wealthy wife and son. enjolras actually isn't his legal surname; it's tholomyes, but when he moved to Paris for his studies he starts going by his mother's maiden name to disassociate himself from his father.
enj starts going to the Luxembourg gardens to read because it helps him focus and he sees Cosette, fresh out of the nunnery, who looks scarily like him. cosette, when Valjean turns away for a moment, asks him a question about the book he is reading. He answers kindly and plainly, and Valjean and Cosette walk away. They keep happening across one another, and Enjolras is charmed by Cosette's curiosity. They talk when they see each other, share books, and discuss the world. Cosette is far more politically conscious than she is given credit for.
there is great angst potential in them never finding out they were siblings, yet valjean watching a boy with the same face as his daughter die on the barricade. even greater angst in cosette learning of her biological parents after Valjean's death, looking into Tholomyes, and learning that he had a son in Paris. She then connects the name to the boy Marius spoke about, the one who would talk to her in the gardens, and discovers only after his death that Enjolras was her brother.