hmmmm..... I don't think I'd accept a booking like that. Definitely insist on each family group having their own rental agreement, with checks in between stays so you know which damage (if any) is attributable to who.
My cleaning lady asked me to consider offering mid-stay cleans (every 2-3 weeks) to keep possible mold in the bathrooms in check. It's a great idea from a cleaning perspective, but also gives you/your cleaning person regular access to the condo, making sure things are okay. Of course, there is the added cost, but you could require them to pay for it up front, or just build it into your pricing.
I personally am moving away from these longer stays, particularly in high season, as I can do so much better on shorter stays (I charge all guests a departure cleaning fee, regardless of length of stay).
Some of our longer stay guests have had guests come and join them for a week or two in the condo, typically family members. I've found out about these AFTER the fact (as guests tell me how nice it was their daughter visited them etc). Hasn't been a problem so far, though, and I have reminded my guests they are responsible for THEIR guests and damage they do in our condo.
Make sure you are not running afoul of local rental laws. In some states/localities, any rental over 30 days is not considered a vacation rental, it's a regular rental and the renter might have special rights that require you to give notice to evict them. There may be a host of other regulations you have to follow for longer term rentals like registration with the city or city inspections.
And if you go ahead, follow MauiOceanView's suggestion about the cleaners. If nothing else, it will keep your cleaners happy with you, always a good thing!
We had an older couple rent our house for 6 weeks during high seasons. During their stay they would host their daughter, son-in-law and grandchild for one of those weeks and then their son, daughter-in-law and their child would come for one of the other weeks. The main point was that the renting couple was always there and so it was all fine.
Three months is a long rental time and encompasses several school holidays so we can see why you would be concerned. If the original couple was always going to be there, it might be do-able but you can't really police that, even with an extra visit from your cleaner (a good idea). If various family members are coming and going, who is going to be the responsible party? We would probably turn this down too as it sounds like a possible potential nightmare. It's tempting to take a 3 month rental, but can the guests give you the assurances you need that your house will be well treated? Guess you could talk to them or just let it go. Hopefully you will book those dates in shorter pieces anyway. Good luck!
Thanks for all the input. Yes, I 'd prefer NOT to rent for long periods, and hopefully won't have to, since this is a shaping up to be a good season for me. Besides, off season is my time to work on & repair.
Good advice about the over 30 days tenent laws. We've had other "normal" rentals over the years, and you have to really understand the local restrictions, obligations, etc.