I'm a US citizen, recently found out my permanent resident parents are not really my parents. I want my biological mother/siblings to be able to visit me. I wasn't really adopted, so there are no records of this. Will this be an issue if I petition for them without proof of relation.

You do not sponsor visitors.

They apply for a visitors visa, if they need one to enter the US.

B2 Visa.

If you know who they are wouldn't DNA be proof of parentage? That might be enough to get the correct petition.