This sounds very inconsiderate, and I would consider parking the main issue with rentals in general. First of all, I don't know what state you reside in, but some states do have laws concerning short term rentals, and some states make no distinction between short term rentals and more traditional leases. So, since Jan may be breaking the law that would be the first consideration. Find out what protections you are provided in your area.
If you live in a HOA, there may also be laws that govern what activities can take place in a neighborhood. Some neighborhoods, for example, expressly ban all rentals. Some have rules about street parking. Some have rules about commerical activities in the home. All of these regulations are areas that could give you a little something to stand on.
I would also let Jan know you have a problem, and ask her to consider your concerns regardless of the laws that may or may not be on your side. It would not be hard for example, for her to stipulate where people should park. As a host, I do this. There is no law preventing street parking in the neighborhood, but I ask guests to follow the posted speed limit, to park on my driveway, and to park any trailers and RV's in a designated area as part of my house rules. I consider this a common courtesy. Many professional landlords get vehicle plates as part of their screening and stipulate a maximum # of vehicles as part of the rental process. Again, this is not because there is a law against having 10 vehicles, it is a courtesy.
Being a good host is just as much about being considerate to your neighbors as it is about being considerate to guests in my opinion. I am sorry you have this problem, but I would encourage you to seek resolution with your absent neighbor or her agent. You may also approach the guests, but knowing nothing about them or the interactions they have with your neighbor/her agent, it is probably best to start with Jan.