Re: Brother wants to marry his girlfrend and let her make life and death decisions...
OP, I'm so very sorry for what your family is going through now, especially your brother himself. A very scary time to be sure, for everyone.
That said, as a woman with inlaws who like to "voice their concerns", I caution you. If you do ANYTHING but offer support and practical help, you WILL push your brother away, and ultimately damage your relationship with him.
As hard as it is to hear this, it is not your, nor your parents business whether he marries his long term partner and mother of his child, or how he words his will or other end of life documents. It just isn't. IF he asks for your opinion or advice, then you can suggest some alternatives, but you mustn't go to him and offer unsolicited advice.
You and your parents can either rally around him, his partner and soon to be baby and celebrate their wedding and the birth of their child, enjoying this wonderful time as a family, or you can push him away and alienate him from you all, when he needs you all the most.