If my sister's name is on my parents checks written right after their names, does that mean she is a joint owner? Will the bank tell me?

My 2 sisters have colluded to try to steal all of my parents assets and forced my dad to sign over $500,000 abt 8 yrs ago and 4 yrs ago, but I didn't knw til now...My dad is now almost dementia, doesn't walk and no longer cares, in fact because my mother has been funneling $$ to help her favorite child, she and that sister use the other sister who controls all the whole life ins. So both sisters are on the take, they are also on the poa's and bank accounts.... My dad was the most reasonable but now he no longer is... If I bring an attorney in now it will just piss my parents off, as mom still wants to funnel money to c and dad just hates attornies, have tried to talk sense to them and mom's best friend went over and tried to today but my mom just lied to her to hide what she is doing?? Mainly tell me abt the bank accts, cuz I think my mom is lying abt that too