maybe michiru and shio not siblings by blood relatives, how family of lion can give birth ...

I think that's what the father's conversation on the phone was about; it didn't seem like that was the case. Maybe he turns into a lion when he becomes more mature or he's like a partial lion due to being half human which turns out to be an African wild cat. Hopefully the upcoming chapters will shed some light (●￣(ｴ)￣●)

For me, that cant be possible in the logic of werecats world that sensei has shown until now... for a werecat to be able to convert, both of his parents have to be same werecat species, or, one of them full werecat an the other half werecat... other way, they cant tranform full or not tranform at all (Like Ei, from the kuroneko series spin off, he cant transform at all, but he has werecat blood)

Michirus "mom" is human and shious father its lion, if michiru were shious father son, he has to be at least half lion, but his another different species... he can transform full, so both of his parents are african cats or at least one of them full african cat and the other half african cat...

follow the logic of sensei in her other works, michiru isnt shiou's father son, and that human woman isnt his biological mom, just his adoptive mom... he's adopted, maybe shious father knows that he was a werecat, but not what type, he was hoping for him to be a lion, or at leats a rare mix, but he was a simple cat... if michiru were his son, there's no way he wouldnt know what type or werecat he would result, being his father, knowing what type of werecat were his mother, but he was surprised when he found out about michiru true nature, so he just lost all interest, as Kategora saids, that why michiru its ignoring him...

There's no blood relationship, so i hope that sensei draw another series of them falling in love, because im almost 80% sure that they arent brothers at all...