You should double check the actual wording of the covenant, as 'Single family' doesn't sound quite right - it may be more like 'single dwelling' which has more to do with not dividing the proerty into several smaller dwellings, and less about the actual relationship of the people who live there. If they are living in the house as a single dwelling but as unrelated flatmates, I am not sure there is much that can be done.
You sound like your main concern is actually the fact that they have 5 cars, which could just as easily be a problem if a single family lived there!