So...

I want to know the answer too

so many unanswered questions...

They aren't blood brothers. They were half brothers by their father who is a lion. Since....fuck I already forgot his name, anyway let's call him mi-chan, was actually a wildcat not a lion that means he has no lion blood in him. Meaning he could have a lion parent. And thus, they are not brothers.
But as to who his parents really are remains a mystery.

Ohh i see~ and yeah parents remains a mystery ╮(￣▽￣)╭

Half brothers are still blood brothers, though? They just have different parents (apparently).

Yes half brothers are still blood brothers cause they share one parent. They Orginally were half brothers being that they shared the same dad. But it was shown the their dad wasn't actually mi-chans dad so they aren't really blood related at all.

Nah I'm pretty sure they are legit half blood brothers, since michiru's mum said in the last chapter "that pest of an old man is part thingy-ma-jig wildcat". So they're blood brothers, they just got different genes from their dad

well there are not a ton of cats and in Kagami brothers story you find out that a lot of cat families intermarry to keep the lines strong. So he very likely has a wildcat ancestor since it is unlikely they were able to always find another lion carrier to hook up with.

They pretty much operate with this "a cat gene is better than no cat gene" ideology with the arranged marriages. As 2 cat genes will almost always produce a cat, while a HumanxCat is a toss up.