What loose ends are there left besides Mei and Yuzu getting together anyway? From the beginning, the major problem was Mei's issues with her father and they've solved that. Sure, there is that issue of "oh they are sisters, this is wrong!", but that doesn't need a love triangle to solve. I think it should have ended a chapter or two after Mei patched things up with her father. Mei really warmed up to Yuzu by then and I think a couple of chapters solely focusing on their relationship WITHOUT 3rd party characters, would have wrapped up everything nicely.
Now you're oversimplifying things here. Mei's issues with her father were important but far from being her major problem, that would be her lack of direction and determination. She does what people expected from her, not what she wants.
Having Mei suddenly become all smiles and hugs would be bad writing since it doesn't mesh with her characterization.
The fact they are stepsisters isn't an issue either, Yuzu's just worried that she's taking advantage of the situation to get closer to Mei.
Other thing worth noticing is that the series now has an ensamble cast, and while Mei and Yuzu are the linchpin that brings everyone together the remaining character are as important like them. With that in mind, we still have loose ends regarding Himeko and Harumin (Harumin's sister being the most intriguing so far).
The series has also adopted a serial format, and new characters serve as motivators to explore new angles with the cast while also causing development for them, therefore, I doubt we will see the ending any time soon.