This summer I drew the short straw and am stuck going on vacation with the sister-and-brother law. Being that our total resort charges will be several thousand dollars, I was thinking about seeing if I can find discounted Marriott GC's at BJ's, Costco, etc. to pay for everything. Assuming I can, is there any reason not to go this route? I'm only worried about some obscure MR provision that would, say, void Elite benefits, make the stay ineligible, mandate my sister-in-law's lounge chair be constantly next to mine, etc.

Thanks in advance for your insight.