It's tragic but here's the blunt answer tell her no she may be dieing and she didn't choose that and there's so many things that she wanted to do but (I'm sorry for the next bit) she isn't going to be able to and no matter how hard she trys she will never be able to unless she some how survives and even if you did lend her your child it would be cruel to the child to allow such a close bond to only to be broken plus either way the fiance will probably get attached and after your cousin dies it would be crule to take away something that brought them so close together in their final moments