I am experiencing this at the moment. It was first picked up at about 25 weeks with an AFI of 26 but over the past week I have 'popped' in a big way. An unplanned visit to my OB confirmed that at 27 weeks I was measuring 37 - 38 weeks. An emergency US was ordered and the AFI was measured to be 29. Waiting for something to happen, so I can so empathise with your SIL.

Like her I'm been monitored very closely. Out of interest is your SIL in hospital or allowed to be at home. I have had to beg to be allowed to remain at home as I have two small children whom need their mummy for as long as possible.

Thinking of your SIL and hope she is not too uncomfortable. Not being able to breath is not nice at all.