Weird. I never get caught at home. Always seem to have a good relationship with the stepdad and stepsis for whatever reasons. Well, I always let her tag along to the mall, so thats probably the reason why she likes me. No idea as to why the stepdad likes me.
There's a lot of little things that affect the chance. If you bring a guy over the house with a bad or nasty personality (I know this is kind of hard to tell at the moment), or if the guy is really old, basically your stepdad will have a higher chance of disapproving of him, and the chance of him interrupting will increase. If you wear a bunch of slutty clothes around the house, don't contribute to rent, etc... over time the chance will go up too.
And yeah, letting Alexia tag along, buying her coffee sometimes, stuff like that, will make her slowly like you more over time and will contribute to the lower chance. There's going to be a lot more control points to these relationships later, where you'll be able to intentionally hang out with your family members, and maintaining a good relationship will have all sorts of little positive effects here and there. For instance now if you have a good relationship, they have a higher chance of being supportive when you're pregnant and making the process mentally easier on you.