I assume you are talking about a transfer of points into your account.
You are only allowed one points transfer in or out per use year. (So doing this for the cousin somewhat limits your account, plus I think the total points in your account, current, banked, borrowed, and transferred cannot exceed 3x your points)
Per the member agreement it is actually against the rules to recieve compensation for a points transfer (so never mention that to MS)
As far as I've read, any points renter can have
DDP added to their reservation, so that is not a problem.
If you go through the transfer, you will have full control of the points, and I'd probably feel better about that (knowing someone else cannot pull their reservation) but make sure your cousin undersands a transfer does not equal a reservation. There is the chance that you could receive the transfer but not get the reservation. If the cousin "rents points" they are actually renting a reservation....