What would be the halachic guidelines for me (a 19 year old girl) being alone with my stepfather in the house? Is it ever permissible? During the day, at night etc.
There is a prohibition of Yichud with a step father, both by day and night. As with all Yichud, there are a number of ways to avoid this prohibition. Including: having a shomer present, when someone may walk in any minute, an open door that someone may enter in to.