Let's assume Mc gets close to the other circus members; how would they react if Mc called them "brother"? Or if Mc told them they saw them as brothers?
(My English so bad sorry TnT)
With love to the whole team ♡
Pierrot would be visibly distressed, trying to correct you while frantically coming up with strategies in his mind so that you will see him as more than a platonic figure.
Harlequin would be confused, then laugh, and taunt you about why you would see him that way or how he can 'easily' make you see him as something else.
Doctor would chuckle and pat you on the head, calling you cute. He'd love to be a big brother to you.
TicketTaker would be horribly confused and wonder how the hell you ever got so comfortable with him to call him such a name. He would politely ask you not to refer to him as such.
Jester would also be confused, likely ask you why you'd call him such a thing and then be sure to let you know that he is NOT your kin.
NOW---if you were to pursue a friendship ending, the protagonist ending in the game (no romance), these answers would change somewhat for pierrot and harlequin at least---those two would then be more accepting to such a title. HERE^^^ I'm just going by how they see you NOW in the game.