In addition to the suggestions here, have you considered asking your mother to be tested also? Your tree shows that she is living.
It may help you figure out your closest relatives in large databases if you have the closest relative to the mystery person tested.
Also, collect any family stories that your family will share. I have worked with people whose siblings knew that they had another father but had never told them before the DNA test.
Please be kind! Not everyone will be happy about old surprises.
Best of luck!
Peggy Mosinger Freedman