Here is a good example of having to decide what is more important to us. It is entirely possible they have scheduled let's say 3 second visits. If you cancel for tonight and attempt to reschedule you run the risk they just won't bother and go with one of the visits in the next two days. One of the reasons I try to make myself available when I have a space to interviewing as soon as someone calls even if next week would be better.

I think what is getting clouded here is that you are busy this week and had time for them last week but they didn't call and will have time for them next week but that will be too late. It isn't their fault they want to come this week. It is up to you to decide how important meeting with this family is. From the initial interview think about how interested you both seemed in each other and go from there. If you have other options and can afford to let it slide then maybe that would be best. They didn't leave themselves a lot of time so guessing they have decided on you and may even be coming to sign papers and deposit in hand.