I am worried that you may be setting yourself up for some disappointment and resentment if you look at this in an abstract host-guest situation.
What you have here is your immediate family coming out for a visit. You are an adult, and it's your new city. But my guess is that they aren't going to think of themselves and you as Guests and Host -- they're going to think of themselves as Mom, Dad, Bro, and Little Sis and you as Big Sis. The family dynamic will not be that different from any family vacation you take together. I don't mean that you will be treated as a child, and certainly not that you should be -- I mean a family of adults. Even if none of them has ever visited California or your city, they probably won't feel a need to be "hosted" as if you were just an acquaintance. So even though you live there and they are visitors, I suspect that they aren't going to be depending upon you the way they might if you had moved to a country they've never visited with a language they don't speak and a culture they don't understand.
More likely, in fact, they are going to want to treat YOU and your boyfriend to some nice meals and activities. I know that's how it is in our family, both when my husband and I visit our adult children in their cities (even the one in a foreign country) and when my mother and big brother visit me here -- and I'm 56, so really, it's not about being considered a child. It's just a family being together, and this time it happens to be in your house.
So I hope you will remember that and not expect everyone to somehow be in stiff, unfamiliar host-guest roles all of a sudden, or disappointed if they don't sit back and let you be tour director. More likely, the dynamic will be pretty much the same as when you visit them. The main thing is just having a good time together as you always do.
I have a lot of house guests, running the gamut from family to friends and even strangers. I can tell you the most important thing I have learned from experience about being a good host -- and serendipitously, it also is what makes it a lot easier and stress-free for me:
Let them help themselves! Coffee, snacks, drinks, extra towels and toilet paper, anything else you can think of -- show them early on where everything is and tell them to help themselves. That will make them much more comfortable, and, once you let go of that odd feeling of being out of control of your own home, it will make your life much easier, I promise. I know it's an adjustment, but believe me, it's worth it all the way around. As this is also your boyfriend's home, discuss it with him first. Just put away anything you don't want eaten/used/observed.
I would do this with breakfasts, too, for your sister (and the others, if they will be eating breakfast at your house). Don't ask -- just put out a bunch of stuff and let people get their own when they are ready to eat. You probably have a pretty good idea what they like, but when I have no idea, I just put out coffee, juice, bagels, butter, cream cheese, jam, cheese, yogurt, cereal, and dishes and flatware. I might offer to make some eggs or something if it's convenient. But I find that people really appreciate not going through a conversation about breakfast first thing. And go ahead and put out some of your healthful yummies, too!
Other than that, though, I agree with the others about food: don't try to educate them. (I'm guessing you may already have had this conversation with them anyway.) I'd say that no matter who the guest were; a good host provides what the guests will enjoy, not what the host thinks that they should enjoy. Just like gift-giving.
Have a fun visit with your family!