Hi and welcome.
Depending on her physical size and the size of her heart, they may have been able to use a valve that may not need to be replaced later on. Someone posted a while back (and I apoligize because I can't remember the specifics) but it was a child that they were able to put an adult size valve in. At your sister's age, a tissue valve most likely would have been the last choice, because that definately would have needed to be replaced.
She will now be living her life on Coumadin. While it is a serious medication, it is one that can be lived with fairly easily. I highly encourage you and your parents to visit the web site
www.warfarinfo.com It is a web site run by one of our best loved and most appreciated members, who is an expert in the management of Coumadin (warfarin). Some doctors will pass along a lot of very incorrect information on this medication, so I highly recommend you or your parents to post anything they tell you here, and our many people (including myself) who live with Coumadin will be able to let you know if it is correct information.
One thing your sister, you and your parent will learn is that it's best to question everything a doctor or medical professional will tell you and then research it to see if you agree. We talk a lot here about being your own best advocate. Knowledge is power over your own health issues.
Your sister should be able to lead a fairly normal life. She will need to take some precautions, but she should be able to participate in many regular activities. I'm afraid that dodge ball is probably not going to be a possibility. But softball, basketball, riding bikes (with helmet) and many other activities should be within her realm of possibility. We have members here who do triathelons and marathons!
Your sister is very lucky to have you on her side. When you get the chance, we'd like to hear just how your sister came to have a valve replacement. Invite your parents to join in here too. When things like this happen, it's very comforting to know others know what you are going through.