Two sets of guests; one reservation for one room

Most hotels require all guests staying in the room to be registered, and will charge extra if more than the set number of guests is registered for the room.

So you could register your sister and (I presume her) husband, however you could likely need to pay extra for the 3 guests - even though there will not be 3 guests there at the same time.

Alternatively you could simply not tell the hotel. In general the hotel will not check the ID of guests (and in fact, in the UK there are laws against them doing so!), so the only time you may experience a problem here is if they were to lose a key and require front-desk assistance to gain entry to the room.

Personally I would suggest a middle-ground - add EITHER your sister or her husband to your reservation. This way you will not be going over the allowed number of people on the reservation (2), but they will still be able to handle any issues as one of them will be registered to the room. You are probably better adding your sister's husband rather than your sister, as that way there is one male and one female, and they will simply presume that your sister is you (the alternative would have two females registered, which may raise issues if they notice a male is staying in the room as it means he's obviously not registered).