DH assures me that he has already told MIL that we cannot afford to contribute. SIL cannot afford to contribute either (as MIL always reminds DH), so really, why should DH contribute....

And I agree, if MIL has that much for a wedding and has a need to replace the tractor due to the wedding, then she can come up with the money. My thinking is that they do not actually need to replace the tractor immediately and could spend time weighing options, including a more detailed cost-benefit analysis of replacing the tractor. For example, how much will it cost to pay someone to plow the snow 2-3 times per year or smooth out the dirt road once every 3-5 years...and based on those figures, how many years would it cost to recoup the investment of a large piece of equipment that will sit idol most of the time. 

Exactly! But I also think MIL should explore other options, since she's not going to operate the new tractor on her own. There are costs associated with DH going up there again and again to use the tractor to plow or do whatever MIL desires at the time...his time is not free, even if MIL doesn't pay him for it. Might be worth talking to someone to see how much it would cost to do whatever plowing, landscaping, earth clearing MIL has in mind, instead of rushing out to buy a new tractor. 

I will also add that the original cabin on the property is in a state of disrepair, since the same elderly uncle that broke the previous tractor under the guise of "repairing it" (DH has used it many times in the last year and did not notice anything that was in need of immediate repair) is also in charge of maintaining that cabin. MIL's cabin (which is more like a house in the woods), does not have the right set up for a wedding (doesn't have a big flat area) and also just looks like a house in the woods, nothing to write home about. MIL loves to make a spectacle, so she will likely want to completely renovate the entire thing...either way, I hope to sit back and stay out of it...and I hope DH also remembers that he is not MIL or SIL's handyman, or contractor, or anything else to get this wedding off the ground. 

She was home two weeks ago (which was kept a secret from everyone, but for some reason MIL told my mother) when she apparently looked at venues. 

Strangely enough, we are not supposed to know she is getting married (MIL only told DH due to the need to replace the tractor) and are supposed to act suprised when she tells us. 

MIL moves heaven and earth to make sure that SIL's dreams are realized, so SIL will never realize how unrealistic her plan is. SIL is also always "too busy" to ever respond to DH (or even sometimes to MIL), so she wouldn't trouble herself with the planning, anyway. For DH's and my wedding, MIL bought SIL several dresses to choose from, enough to cover the gathering two days before the wedding (stand-in for a bridal shower), the rehearsal dinner, and the wedding. Then MIL arranged with my mother for SIL to have her hair and make-up done by my stylist (who is also my cousin). All SIL had to do was get on the flight MIL had booked for her. 

I would agree with you if it was not MIL doing the planning. I actually planned my own wedding in eight months, but that was possible because I was not overly picky and didn't have grand visions of what I wanted. MIL is obsessed with appearances. For my wedding, my mother ended up planning the rehearsal dinner because MIL wanted me to basically plan a second wedding by renting a "historic home" and shipping in food from DH's home state...not sure if this was just her way of getting out of planning and paying for the whole thing (because my mother found and reserved the space, she also paid the deposit). MIL is also terrible at actually planning. To add to that, the family cabin does not currently have a suitable space for a wedding for a couple hundred people. This is why MIL is insistent upon buying a new tractor, so they can clear land. One of the areas she's currently thinking about is currently covered in thick weeds and overgrowth. The other is covered in random junk acquired over the years by elderly uncle. The cabin is almost three hours away from our home, so it's not as if DH could go by after work. It also gets heavy snowfall in the winter (the area has already had its first snow), so the earliest they'd be able to get in to do any land clearing would be late March / early April. That only gives them a couple of months to get everything set...and I will not be happy if DH is expected to spend every weekend out there prepping the space...or worse, take unpaid time off work to do so. BIL won't do it because his kid is now on a travel baseball team, so he'll be off at baseball games on weekends. 

DH contacted someone about the existing tractor. The problem is that the parts elderly uncle removed and either broke or can't put back in or not readily available because the current tractor is so old and also are parts that would more easily break down, meaning tractors of that type in the salvage yard don't currently have any in good enough condition to serve as replacements. 

Yes, FIL passed away over 20 years ago, but did leave significant money to MIL. 

DH's family always has this weird tendency to assume certain property is "co-owned" when it is time to repair or replace, but not co-owned when it is time to use or benefit from it. One of the problems with the old tractor was that elderly uncle assumed it was his outright (in part, because FIL is no longer around and DH's grandparents are also no longer around). He was super possessive about who could use the tractor and was constantly tinkering on it (which is why it is broken), but as soon as it needs to be repaired or replaced, it belongs to "everyone". We've already paid a couple of times to have someone repair pieces that elderly uncle has removed and couldn't figure out how to put back in or that have legitimately needed repair. 

DH sees it as his familiial duty to "maintain" the family cabin, including the stupid tractor...and to further complicate matters, they buried their father's ashes on the property, so he has an extreme emotional attachment. He is very driven by what his father would have wanted, to the point of it being something other family members take advantage of, IMO. Also, in many ways, MIL expects DH to be the stand-in for FIL in his absence (even though DH has another brother, the golden child, who everyone says is just liked sainted FIL, but is never really expected to take any responsibility). 

Haha - love it!

And also about everyone "chipping in"...when really, they could pay a couple of thousand dollars to have the place plowed a few times a year, decide that a wedding on the property is too much work, so SIL will have to be sad, but will survive, and that buying a replacement tractor is not a good investment. DH's family is terrible about planning and seems to make decisions based hopes and dreams, rather than reality. I'm a planner and also a realist, so it drives me nuts!