Is Dave acting like a fourth roommate? If he's eating, sleeping, using facilities in the house on a regular basis with great frequency, and if he spends an inordinate amount of time at Steve and Michelle's more than he does at his parents' house and stays there in such a way that it is like a second home to him, then that should be taken into consideration. For example, if he goes to Steve and Michelle's house after work (assuming he has a job) and stays there instead of going to his parents', uses their showers/ bathrooms, eats food from their fridge, etc., then those are all things that may be just details but are still important.
Depending on the amount of patience that Steve and Michelle have, some of the things listed above may or may not matter to them as much as getting to know Dave first, which a lot of people have already mentioned. Steve and Michelle say he doesn't seem rude, inconsiderate, or offensive, yet they also say they don't know much about him, so Dave is basically unknown; he may or may not be those things according to their knowledge.
This problem seems to have been compounded by a lack of formality, however unintentional it may be. Janet did not ask for permission, and Dave has not introduced himself yet. In Janet's defense, it sounds as if she's been through a lot in terms of relationships, so maybe she neglected to ask out of oversight and not as a way to take advantage of her sister's hospitality. Dave may or may not be a decent person, so I would approach him evenhandedly without giving him undue credit or notoriety to his reputation at this point. If he's a decent person, he might be a shy or socially awkward guy that lives with his parents. If he's not a decent person, he might be an irresponsible man-child that lives with his parents. Or a mixture of both scenarios. If he keeps avoiding initiating a conversation, it will be difficult to tell obviously.
There are possible solutions to consider, though I do not guarantee any of these will work. A better outcome might occur if you use all three together to some degree.
A. Steve and Michelle may attempt to get to know Dave better by inviting him and Janet to lunch/ dinner or a similar social event. The possible downsides of this option is that the meal will cost money and time, and if Dave still does not talk much, the conversation will be walled by long awkward silences with nothing still known about him. The possible upside is that if Dave does open up more and seems nice, Steve and Michelle might feel more comfortable with his presence. If he is crass, rude, stupid or has similar negative characteristics, then the meal is still a success because he will have revealed a part of himself previously hidden. If the latter were to occur (he shows an ugly side), Janet will be present and will be hard pressed to defend his character, either to herself or to Steve and Michelle.
B. Offer Janet increased emotional support, more than usual, like stutters said. Janet is likely emotionally fragile from the disappointing relationships in her life. Instability creates fear and thoughts of inadequacy. She may like Dave or think she likes Dave to an extent that she perceives being with him as a way of being happy in the way many people do their own relationships. If she is given enough or more love from a different source, such as other family and friends, two things may occur. If she grows more confident and feels supported by others, she may feel like she deserves better and dump Dave if he is no longer the primary or sufficient source; she might now see flaws in him or recognize new ones she overlooked while she was in "love." If she still sticks with him but is happy from the love she feels from others, perhaps Dave genuinely pleases her. Maybe.
C. When Janet gets back on her feet financially, and if Dave does not have a steady job or a nominal paycheck or is unable to move in to a place with Janet when she inevitably leaves her sister's house, conflict may ensue and lead to a breakup. In this case, no one really has to do anything, but this process will take an undetermined amount of time. (It would be best to figure out if Dave is a potentially dangerous guy beforehand, so you can prepare for the worst when the two split. A form of Option A.) On the other hand, she might become the primary earner in their relationship, but the previous relationship where she had $7000 appropriated from her may have taught her to be more cautious concerning matters of money.
D. Do what others have suggested and clarify and enforce a set of rules. Ultimatums can be ugly but necessary depending on conditions.
None of these options is an absolute solution or an infallible litmus test for Dave's personality; if he is a psychopath (which is a very, very, very slim but still possible chance), then I have met my match and can't really offer any useful advice to you. There's always the possibility that he's an average, nice guy too. Who knows?
Of course, you could always ignore my post.