Welcome to FT and the BA Board, springy

The official answer is 'no' - although as a Silver you are allowed a guest, that guest must be on a departing BA flight (or the same oneworld flight as you). They will want to scan her BP as she enters the lounge, and she won't have one. That said, if you politely explain and ask the lounge desk agent that you just want to spend an hour catching up with your sister, etc. they may turn a blind eye...but don't be surprised if they say 'no', it would be the exception not the norm to say 'yes'.