OK I need to know for sure if my siblings were legally adopted by their step father. They all have his last name, but their mother could have just changed their name. I'm wondering because well honestly my dad has dementia. And is going crazy. Slowly losing his mind. Well for legal reasons and just because I'd like to know if my brothers and sisters are really my siblings. And if so. We need a probate lawyer and will made and all that. If not, then, legally its just me. And also, I don't think so, but would my half sister from my mom have any rights here? Since he did raise her.

A lot depends on exactly where you reside... as laws vary from country to country and even state to state. And also if there was a will created before he developed dementia.

If your dad has a will and leaves them a share in the will, it does not matter if they are adopted or not.

If there is no will, you can hire an attorney and challenge their claim. And produce court records

If they were legally adopted, there has to be a court record of the adoption. No court record, no adoption.

As Chuck said, if there is a will and they are mentioned, then that makes an adoption moot. If there is no will and it is no longer competent, then it is too late to make a will. Absent a will, only legal children are entitled to inherit.