I think he takes money from the seme because he doesn't go home so he has no one to give him allowance. When he ran away from home the uke used the seme's guilt to make him give some money to the uke. And I think that's why he takes money from the seme.
I wouldn't call it ‘replacement setting where in the end seme falls for the uke himself and not as a replacement’ because he never thought of the Nazuna as a replacement. It's what Nazuna keeps telling himself because in his mind, he thinks the only reason why Matsuba cared for him is his face but he was actually lying to himself because without that and the cash, then he has 'no use/need' and he admitted in the end what he wanted was to be in his sister's place. He liked Matsuba even when his sister was alive so the death made him feel guilt and those last words twisted him. That's why Matsuba was so concerned when Nazuna visited him that first night they slept together.
Nazuna’s story is very....sad, like really sad. He has been pining for his sister’s husband, then his sister dies leaving behind words that were interpreted by nazuna is a very weird way, nazuna then tries to fill in the void of his sister adn believes that there is nothing more to him than being his sister’s twin T_T the plot seems basic for a ‘replacement setting where in the end seme falls for the uke himself and not as a replacement’ but ion get the money part? Can someone explain why he takes money from the seme? Im happy it ended on a happy note considering how heavy the manga started. And the oneshots? Ye...lets not :)