If she has a visitor visa it can't be extended beyond six months. She must either leave the UK or, if appropriate, apply for a different category of visa.
If your sister-in-law has a visitor visa valid for less than 6 months, she may be able to extend it up to the 6 month limit. If she's got a different category of visa altogether, she may be able to extend it beyond the 6 month limit (which is only rigid for visitor visas).
Start here for information:
http://bia.homeoffice.gov.uk/ukresidency/exten dingyourstay/
If you get confused (which is easy to do on that website), phone the UK Visa Services helpline on 0845 010 5555. (Unlike many of the expensive, commercial, so-called visa helplines you'll find advertised on the internet, that number goes to the Foreign and Commonwealth Office and is only charged at local call rates).
Chris