On the DVC points for rent board they usually trade/retail for about $10 per point. But that is the retail price,
If you'd want to cover the dues part of the points ( your SIL may be a long time member and feel the points paid for themselves a LONG time ago) the dues are about $4 per point. The "dues" if you're not aware are the ongoing costs of the resort and each member is charged an annual fee for their share of the running cost of the resort. It does vary from resort to resort, but $4 is a close enough guess, some resorts are a few cents less, some a few more.
If actually getting them to accept the money is a problem you could always take them out to dinner (if you live nearby), again I'm sure it's helpful for you to know a realistic cost for those points but as a best guess on it I'd say somewhere between $150-250 in the situation you've described. Obviously there is a difference between cost and market value. IF your SIL wanted to rent those points out to a stranger ( and she probably wouldn't want to) she could earn between $350-400 for those points, but there is hassle involved in it and she may just be happy to share out her good fortune to her relatives.
Just as a "for your info" if you could change your plans slightly, for the same number of points ( or indeed fewer) you could stay for Sunday, Monday and Tuesday nights. Friday and Saturday nights are 2.5 times more expensive than a weekday. It would be no difference in booking time/admin/work for your SIL, would be a few less points used, and you'd get an extra night accommodation out of it.