For TS security check, what do the FBI investigators ask about?

I've been tentatively approved for a top secret security clearance for a military job, and of course, now I will have an FBI investigator assigned to my case and he will go out and check out records from as long as I've been alive. I'm confident that I will pass, but I'm quite curious what he'll be asking about. Please don't say ';everything';--answer each of these categories if you know. I know that he will research my encounters with police and my credit history.



Will he research my extended family history?



Will he research my medical records? I've heard they don't unless they get an indication that I've lied, such as when a recruit hides a condition and dies from it in basic training. I wonder how this ties in with the TS background check, though.



What if he interviews someone who speaks poorly of me unfairly? What if they lie about me?



What if nobody at one of my former residences remembers me?



My brother is always in trouble with the law, but I don't even have regular contact with him let alone a brotherly relationship. Could this negatively impact my clearance?



I think that I've provided everything that was asked for, but what if I've forgotten something? I've heard that he will interview me after he's done all of his field research and give me a chance to explain anything. Is this true?For TS security check, what do the FBI investigators ask about?
Having never been privy to exactly how they conduct a Background Investigation (BI), but having completed Intelligence School AT with an Military Occupational Specialty (MOS) of Intelligence Analyst, I might be able to give you some information about this matter. Part of my military duties included processing of Confidential, Secret and Top Secret clearances. As far as I know it is a member of the Military CIC who will perform a BI on you. He/she will talk to your friends and family, high school and college teachers, instructors and classmates, ask for names of additional people who know you, ask former landlords about you, talk to any former civilian employers and fellow workers, check out your family relations and dynamics, etc. In light of your minimal contact with your brother, this will probably be shown in the BI and not be a cause for concern. If the investigator needs additional info about you or needs to clarify something, he/she will probably contact the initial interviewer to obtain the additional info from you.