Hello Nathanael,
Very nice to hear from another Forbister. I have been doing my own family research and also searching for other forbisters for about four years. I have found that the Forbisters of Australia are related to the ones in Sunderland England, who may be related to the Canadain Forbisters. I haven't found that link yet.
I don't have much information on the Forbisters in the USA. As for your Grandfather, William. I have one William that I can't account for, and I know that he may have moved to the USA. If he turns out to be your Grandfather, than you are related to me. All I need to comfirm this is the names of your Grandfather's brothers and sisters and maybe his parents names.
So far I have 149 ancestors and living Forbisters here in Canada. I look forward to hearing from you again. Take care.
Derek