No, don't pay.  In the US medical care is not provided based on the ability to pay.  She may incur debt but she is not going to pass away.  Also she could wait until she is 65 and use Medicare.  

You are 25 and you have lots of spending, earning and buying left to do in your life.  I am surprised that older relatives would put so much pressure on a 25yo.  That is inappropriate and should be embarrassing for them.  In any case this is not your issue and the older siblings should find a way to pay if they care so much about her (many people supporting would make it much more affordable).