I'd stay at another place just to avoid the drama. 

Personally, I have never understood the destination wedding / vacation with us invite. My son was best man at a cruise wedding. The wedding was very small and the bride's parents paid for everyone's trip. (They said it worked out cheaper than a big wedding at home.) But, my son said that while it was nice that it was paid, it did have it's drawbacks -- the biggest of which was it used almost all of his vacation time and he found out that cruising is not for him.

Weddings, to me, are like family reunions. Family reunions are fine. I actually like them. But, ten days. We did a long weekend reunion once and that was plenty long enough for me. And as the bride, I would not want to spend my honeymoon with family. The thought of it makes me ...... 

None of which answers your question. I would have thought as you did. But the bride and by extension her dad, seem to have originally assumed that everyone would want to stay the whole time. BIL may be financially in way over his head. He may be realizing that not all the invited relatives and friends are even coming to this wedding and he's guaranteed $$$$. Is it right to charge everyone a ten day share of the cost? Not in my world, but in my world I would never have rented a resort either. 

I'm guessing that the issue is that they have booked the entire resort for that 10 day period so the hotel would not be able to book your room for those other days.. since the couple doesn't want others at the resort during that time?  So.. THAT is why they want to charge people no matter how many days.. because they have to pay for the entire resort.. for the duration.

I would confirm that to be the case.  The alternative is that you could respond that you would prefer to book your own accomodations.. because financially the cost of that resort is too much.

Or.. you could send a gift and regrets.

Honestly.. the couple really bit off a lot and made a lot of assumptions regarding how long people would really want or need to stay at the resort.  It probably is unchangeable now.. but what they should have done is booked out the whole resort for the day before..day of and day after the wedding.. THREE days.  They then should have polled guests to see if anyone would be interested in extending their stay to longer.. then a BLOCK of rooms could have been offered for a discount rate.. but the resort would not be "exclusive" for that time.. so there could be other guests.. THAT would have accomodated the groups that wanted to only come in for the short span of the wedding 3 nights.. (rehersal first night.. then wedding.. then one more day for some group activity?).   Then.. those that wanted to stay on longer (or arrive earlier).. could take advantage of a discount to pay for any extra days they wanted.. within a week before or after the wedding!

It is quite presumptuous that EVERYONE is going to be up for taking a 10 day vacation to coincide with their wedding.  It's kind of ridiculous.. because it puts a big financial strain on people that might otherwise not have booked into such a plush experience!

The alternative is that the couple just pay it all themselves.. and host everyone for as many days as they want..   But expecting the attendance to your wedding be including a 10 day stay.. whether you can use it or not? NOPE that is absolutely bonkers.

At every destination wedding I've been to - a grand total of two - the couple reserved a block of rooms (I assume up to a certain date, at which point they were released if not booked by wedding guests), and guests booked their own rooms, for their desired length of stay, directly with the resort. DH and I had looked into a FL destination wedding and that's also how it worked at the place we spoke with.

If you want to go, I'd just book the accommodations you need elsewhere and drive to the wedding - it's crazy to pay for 10 nights when you're going to be there for only a couple nights, and I wouldn't want to be spending my every waking hour with the wedding party/guests anyway.  But IMO it's perfectly acceptable to decline a destination wedding, even of a close relative - that's a risk they take by choosing such a venue.

For 10 days if I was only staying Two nights.   With BM floating around.   I would make other plans.  Air B and B.  That resort but renting my own room for two nights.   Diffent hotel,  

I would definitely NOT want to be at anything for 10 days (unless it was MY vacation) Its too much. A few days - up to 5. Most people only get about 10 days a YEAR for vacation.

This wedding is for folks that dont need to work and have deep pockets.

Id book somewhere else and be done with it.

two nights you want. If that is not possible at the destination resort, stay at an ajacent property and attend the wedding. I would not subsidize the wedding by paying for 10 nights if I was only an in and out attendee.

Nope.

NO WAYYYYY would I stay with the family.  

Find a find a local hotel. Skip the Air bnb. --Far too many odd rules and frankly too much work. Pamper yourself it's good for the soul.

No, no way. Not fair to the attendees. Stay somewhere else like others say.

My DH and I planned a desitnation wedding. Because it was actually easier for everyone to meet where we went. LESS travel for everyone than if we had it here where we live. But it was super flexible, people could come/go as they wanted, stay longer or less time, whatever hotel in the area they wanted to book was good and we had a couple easy get togethers with everyone (two of which we paid for entirely). It was great!  No expectations! 

This expectation is insanity. DH and I had a destination wedding, and I know others who have also. It has always been done the same way- you block off an allotted amount of rooms for say, 7 days (which is just an estimate). People can stay more or less, it's up to them, and once they book their room, the hotel adjusts their "reserved room" according to how long they actually want to stay. You only pay for your OWN room for the number of days you stay. You never expect someone to pay for days they didn't stay! And 10 days? Who expects someone to come to a wedding in FL for 10 days?? However, if you have a block of rooms reserved and someone stays only 3 days, for example, you may not get that room to count toward your block, as sometimes there is a minimum stay. The thing with blocks of rooms- often the bride and groom get freebies for the more people, and the longer they stay. For ours, if we had 6 rooms booked at 5 nights or more, our whole wedding was free. At the last minute, one of my friends had to cancel, so we only had 5 rooms...we ended up having to pay for our wedding. But it is what it is, you can't expect people to pay outlandish amounts of money for your wedding. 

We also put out feelers first and asked who may be interested in coming, so we had an idea. We would have done it anyway, but that helps you pick a resort to have the wedding. 

I would have your DH tell his brother that you can only attend for 3 days, so you'll pay for that, or you will be happy to stay in a different location. His brother should expect this response from more than 1 person.

Never, ever ask for people to pay for a 10 day stay at a resort when they were only there for a couple of days! And I'd never pay that to attend a wedding.  Regrets and a gift,  then I'd plan my own vacation far from BM and anyone else giving off toxic fumes...