Two sleepovers in any one weekend would be too many. In fact, one sleepover in two weeks would be too many for us, and for all the families I know, I think (my daughter is nearly 12). She and her friends very rarely do sleepovers unless it's for a birthday party. A few of her friends who have extremely close best pals, who often live very close in the same neighborhoods, do sleep over more often.
The most basic thing to tell yourself, and then her, is that when a friend sleeps over they are having fun and that's fine, but no matter how well they claim the slept, they are not getting enough sleep and the sleep they do get isn't going to be very restful. One night of that occasionally is great and a treat.
But two nights in a row is enough to get her body especially tired and off-kilter, and will affect the following week at school. Sunday night alone isn't really going to get her body and brain back on track. She will spend all Saturday kind of tired and draggy after a Friday sleepover; do you want her also tired and draggy all of Sunday too? That isn't wise and she is old enough to get that.
Also, if she's doing so well in school (congrats!) she needs weekend time for homework, I'm sure. And she needs to be fresh enough to do it.
I think the real issue here is not so much focusing on a sleepover rule; isn't your real issue that you're worried about her tendencies to stay in her room a lot and think about One Direction? You seemed very thrilled -- maybe a bit too much so -- just to lay eyes on her that one night when she had a friend over.
She might need more -- well, let's not say "play dates" as she's old for that term, but more time with friends just coming over to do stuff. Doesn't have to be a sleepover; can you interest her in some crafts that she likes? Tell her to invite a friend for a couple of hours on a weekend afternoon and you'll provide any and all craft stuff they want (set it up on the dining room table so you can see her and they don't disappear into her room forever). Heck, I'd let them decoupage boxes with pictures of One Direction if she wants; whatever is fun for her and the friend!
Tell her you'd like to take her and a friend to a movie they really want to see.
She likes Wii, right? Suggest that if she wants, she can have one or two or three kids over for a Wii tournament and she can pick the pizza and the dessert (then have her help you make the dessert - time in the kitchen is time spent together).
In other words, find ways to help her think outside the two boxes in her mind: The box where she's in her room and the box where "doing something with a friend" currently means only "having a sleepover." She may need some gentle suggestions from you because she only sees those two ways of doing anything just now. She may at first roll her eyes and think, "Doing a craft, how dumb" or "I don't know three other kids well enough to have a whole Wii tournament" but she will get past that. Don't make her feel you're trying to be her cruise director, throwing a ton of activities at her, but do make some offers without pushing.
She also might benefit from some extracurricular activity. She sounds very typical and maybe she needs down time to regroup (which is just fine, some kids need more alone time than others) and maybe she's not a "joiner." That's OK too. But if she found something that truly interested her, it gives her a new outlet for friendships and developing skills that are not related to schoolwork -- it can be good for kids to have a side to them that is not related to school or their school friends. You would need to do some research about what's available and you too will have to think outside the box to find things to offer her that aren't just the ubiquitous team sports.
Update us. I feel for you. Kids want to repeat what they know was fun, hence her asking for another sleepover. But the whole post makes me think there's the bigger issue of your just wanting to see more of her yourself and wanting her to get out and about a bit more, maybe. She sounds normal and nice....and I remember what it was like in seventh grade to have massive, obsessive crushes on pop stars!