I am working in a hospital, my immediate superior told a co-worker in a sister company hospital re: my transfer to their workplace...I was hired in my current position in my resident hospital. I am not a corporate employee that they can move around to other hospital under their umbrella. can you please give me your opinion on the technicalities or legality of transfer to a sister company?

Unless you have a bona fide, enforceable employment contract (not just an offer letter or similar), they can transfer you anywhere they like. You have the option to refuse the transfer, thereby, effectively resigning.