Try to separate the issues.
1. He kicked her. It doesn't matter if her foot is broken, it's still wrong. (That's assuming that he kicked her in anger/frustration, vs. an accident when they were both going for a soccer ball or something like that.) So there have to be consequences for that. Explain that 3 year olds maybe can't control themselves, but 7 year olds are old enough to use their words, to walk away, etc. So the consequences need to be taking away something that is a privilege earned by older, mature kids (like 7 Y.O.) until he can demonstrate the maturity and show he deserves it. So choose something that is a 7 year old privilege that no little kid would get. Maybe it's video games, maybe it's computer, maybe it's going to a friend's house alone. So yes, he's crying, which shows he either has some empathy (good) or he's afraid of consequences (normal, not necessarily admirable).
2. If her foot is broken, then she is going to have significant pain/discomfort as well as a cessation of her normal activities. Yes, an apology is a good thing (if he means it) but make sure she doesn't immediately say, "Oh it's okay, it doesn't hurt." He should still have to do some things to help his incapacitated cousin - work out with her parents what she needs. Maybe he needs to spend a few hours over there on weekends waiting on her hand and foot - putting a footstool under her foot, getting her lunch & snacks, playing some board games (especially ones that she likes and he doesn't), etc. Let him be just a little inconvenienced since she is significantly so.
3. Expenses. Let him know that injuries cost money. Let him see the bills if your B and SIL will share them. Or let him see the amount that your family owes. Then assign chores that he can do until the debt is paid off. Maybe you split it with him - since of course a 7 year old doesn't have a lot of earning power, and you don't want to create problems by having him think that sweeping the kitchen floor is worth $20. He'll want that much in the future! Even if it was an accident, there are expenses. So he has to help pay for them. Make sure there are some chores he can do decently well, and make sure there are others that are unpleasant. Also say you have to save money for her now, so that means taking away some luxuries from him (favorite snacks, going to the movies, any other discretionary spending). Let him feel the pain.
When this is done, find something to praise - how he stepped up, how he took responsibility, how he grew as a person through this experience, and how he will not hurt someone again. Even if he's upset, that doesn't get him off the hook for doing something to make reparations.