They are only 7 years apart...?
They could very well be couple in the next couple of years. I'm 24 have a cousins who are 16, I7, 18 and they were already taller and had deeper voice than me the movement they hit 15. It's like we're the same age now despite the age difference. (We are all dudes btw)
I was againts there "relationship" till now cause I thought tsubaki was like 7 years old while yuu nee was 16-18. It was creepy. Thank god that isn't the case