I think my dad is hiding something from me

I think he adopted my stepsisters but is hiding that from me, idk like, once one of them asked about when thats gonna happen in front of me and he was like ''thats not possible bc i'd need your grandmother authorization'' which is ??

my stepmom is really nice, but idk i get suspicious of her sometimes could b just me overthinking but idk

im suspicious bc he has the habit of doing stuff and telling them not to tell me, i think he did that but im not sure and im scared to ask