making another reservation but another person paying for it,
 

Hi, I'm planning a stay in Westin Boston with my sister's family, and my sister was planning to pay for it. When i tried to book using my rewards account and I put my sister's card and name it wouldn't let me. How would i go around this? can i book a reservation under my name, but the other guest paying for it?

Add your sisters name as a second guest in the room

You may or may not receive the points / status credit for your stay if your sister pays. Which begs the question of why you don't just open a Marriott account for her and have her book directly. (or you pay and she sends you the money later)

If you have multiple rooms at the hotel and don't care about points / status, you could also just ask the front desk to move your folio charges to her bill.

Are you booking an advanced purchase rate? If not, go ahead and book the stay with your card and have your sister pay down (NOT PAY OFF) the folio the night before check out. Then check out leaving the balance on your card. Some properties get to be sticklers with the member having to pay for the room to get credit per the T&Cs. If your payment is the last one on the folio, you should be good.

If you are doing an AP rate, then maybe book with your card and ask your sister to pay you or directly to your CC?

book a prepaid rate and just enter your sisters card details, if there is an option for the name, just use yours. the hotel doesnt match names.

keep in mind your sister and that card *MIGHT* need to be present at the time of checkin. the rules say you should have the card on you, but I have had the front desk not worry too much about it.

I have done this before. Basically, I made the reservation with my name and credit card info. Then at check in, we just hand the front desk clerk the other person's credit card. Every time I have tried that, it works.