The thought of this just makes me uncomfortable. You know your family situation better than I do, but a
DVC purchase is for a very long time.
With DVC you are buying points, not weeks. You can bank points from one year to the next. You can also borrow points from the next year into the current year. You could just buy fewer points and use them every other year. Yes you'd be paying maintenance every year, but it would be a lesser amount.
For example, say that you decide that for two families sharing you need 400 points each year and let's say that maintenance was $4.00 a point. So each year you'd split $1600 maintenance and pay $800 each.
Instead buy a 200 point contract and bank all the points the first year. Second year you have the 400 points you need. You'd be paying $800 a year in maintenance, the same as if you were sharing. (If you wanted to travel the first year, you could borrow the 2nd years points into the first year instead)
If your SIL is interested, she could buy a 200 point contract as well. Each of you have your own contract. If one wants to sell, they can do so without involving the other.
It is possible to transfer points between contracts (rather complex rules regarding this, but there are lots of posts on here explaining it).
So you can buy a contract that will let you go every other year and will not cost extra in maintenance fees.