I have a good amount of great STJs around me, so I don't have the negative associations that most do. Plus, it probably helps that I don't turn into a passive aggressive weakling with no backbone around them either *eyes rest of the forum*
Dad and Sis-ISTJ
The first obvious difference is the degree of socialization. GF is the quintessential social butterfly and she will talk to a wall. Dad and Sis have lower thresholds for conversation out of their existing social circle.
Second, is priority. Dad and Sis have an extreme tendency to get their environment, or get into an environment, that is as comfortable as possible. For the GF, this tendency is largely secondary and is only pursued as long as "things to be corrected" or "tasks to be performed" in the environment are completed or in the process of being completed. Dad and Sis are content to do nothing and be cozy, GF is not.
Third, obstinance. I actually find the ISTJs to be more "dug in" than ESTJs. For the ESTJs, Si is in support, not in control, so essentially I've found that ESTJs use it a lot to simply discourage time-wasting exploratory endeavors. They are still logical beings, and as such, far easier to convince to your viewpoint provided the structure of your thought is sound and your reasoning proper. With ISTJs, the final stop is their personal experience. Far harder to change and screw around with.
Just thinking out loud. Call me on BS if you see it.