Agree with slappy...there are serious issues to consider when it comes to joint ownership of an asset like that.
I'm making the assumption that we are talking about a single DVC contract. If there are actually multiple contracts, he could give some to you and some to the sibling. Much cleaner there.
If it's one contract and both of you are listed as owners, things like marriage, divorce, death or bankruptcy of one owner could negatively impact the other. I've heard of both divorce and bankruptcy situations with multiple owners where the courts ordered the entire contract sold.
And then there's the issue of annual dues payments and actually using the points. Dues must be paid regardless of the financial circumstances of each owner. If it's your sister's turn to pay dues and she can't come up with the money, DVC will freeze the contract and come after both of you.
I know these are all situations that we may think we can avoid but one never knows what the future may hold--cheating spouse, loss of job, financial troubles, family rift, etc.
If there is only one contract, I really think it's best to leave it to one child. Then maybe the two of you can come up with some agreement for the non-owner using the points. For instance, if you get the contract, she would reimburse you for all dues associated with the points that she uses.