Claiming Younger Sibling

So I’m 21 yrs old and my younger brother who is 16 has been staying with me in my care for going on 2 years, I don’t receive any money from from our mom/dad but yet our mother has still been claiming him on her taxes and won’t allow me to have custody & I’m just now working on guardianship. Which hasn’t been easy trying to get neither. Is it possible to claim him as a dependent on my behalf without it red flagging?