Usually if the waters go at the start of labour, then your baby is NOT in the optimal position: the baby is likely to be in the posterior (back-to-back) position. Your labour will be much longer and more painful.
If you are doing all you can to get your baby into the optimal position for birth, then this is the best precaution you can take for preventing your waters from going at the start - or even before - labour begins.
However, the waters don't smell bad (like urine does) and will, hopefully, be clear, so this isn't something that should be too big a clear-up operation if it does happen.
My waters broke at the start of my first labour and I immediately went into shock. I didn't realise I had gone into shock until my m/w appeared to check on me and I was shivering and my teeth chattering. I wouldn't want to be driving if this happened again. Besides the risk of infection (which can be minimised by staying indoors at home, not bathing and not having sex or inserting anything into the vagina), there's also the risk of cord prolapse or the cord being compressed by the baby's head into the pelvis (usually due to the baby being in the posterior position). I'd be much more worried about these issues than cleaning up. If your waters do go early and you're worried about cord prolapse, get on all fours and stick your bum in the air, and your head on the floor until you have been checked by a m/w.
Lastly, your S-i-L going into labour early should have no bearing on you going into labour early. When your grandmother, your mother and your sister(s), if any, have gone into labour is usually a good indication - but not always - of when you'll go into labour (as is the length of their labours a good guide to how long yours might last).
It's perfectly normal to have concerns about labour/birth and you just happen to have fixated on this particular topic
It's better than worrying about the pain!
now i am thinking.....oopppss I have just got a new bed... 
maybe i might get a mat just incase, nearer the time... 
