If your relationships with your relatives are good enough they feel comfortable dropping in, your relationships should also be good enough that you would be comfortable saying openly, "Hey, we really need to establish a good bedtime routine, and that means Sally's lights go out at 8:00. That means her bath and quiet time before bed starts at 7:00. So if you want to come visit, please always call first to see if we're already on the way towards bedtime, and if it's 7:00 or after, please wait and call the next day. We love to see you, but lately I realized that visits need to be earlier or Sally's very cranky the next day -- you don't want her to be cranky, do you?" Say it with a big, loving smile.
And then: Immediately offer a time they CAN come. "Are you free tomorrow at 5:00? Let's go to dinner." "Why don't you come at 3:30 and we'll all take Sally to the park." "Let's get together for a longer time this weekend and do something special--go bowling, go to a pool, go to the playground," etc. And so on. You are fortunate to have relatives nearby (we don't) and to have good relationships with them, but they also need to respect boundaries -- and if things are good and you put it well, and you are very clear you DO want them to see your child, just not after a certain hour, it should be OK. If you offer them invitations to see and do things with your daughter it should be clear to them that you do want their company but the weeknight schedule is important to her having a routine. If they say it's "like visiting hours at the hospital!" to you, SMILE and say, "How about tomorrow at 5?" or whatever.
I do wonder: Why are the grandparents and sister and child coming over at times that run past 8:00 at night anyway? Do they all work during the day and that's when they're available? If they are retired grandparents, I would think they could come over at other hours of the day. If you work outside the home and your daughter is in day care, heck, maybe they could pick her up from daycare early some days and spend time with her if that works for you and for her. If everyone involved is working days, then you might have to reinforce that the time to see you and your daughter is weekends or weekdays around dinnertime--suggest they bring take-out, or get it yourself, and invite them over, but kindly let them know that "Hey, I see it's 6:45 -- in about 15 minutes I have to get Sally into the bath" or whatever.