I would not try to "talk" DH into anything. Even if you "win", you could lose, overall, in resentment. This is a large $$ purchase, and nobody should feel like they were nagged into it.
That said, here's what worked with my DH: I did all the research and came to the conclusion that
DVC was best for us. I presented the research only--not my conclusions--to DH. Then I walked away. By allowing him to reach his own conclusion, he was satisfied that it was a good purchase for our family.
In addition, you and your mom need to reach a defeinte agreement on how a split contract would work. Would you all go together, or alternate, or some other arrangement? Whose name is on the contract? Who's responsible for maintenace fees? If you're both on the contract, either one can use the points--you should have a firm agreement on what is "fair". What if one of you wants to sell--does the other party of first dibs? Do you need enough points for an annual trip?
Those things should probably be spelled out, just so there are no hard feelings later.
But getting back to your original question, I don't ever think it's a good idea to try to convince an unwilling spouse. I am a firm believer in finding out their sticking points and being able to address them in a positive manner, though!
And I can't imagine trying to hide such a purchase. What would be the point? Who cares what the in-laws think? I just don't get that.