There's no need for you to become her brother (and this is not really possible anyway - the only way this could be done is if your parents formally adopted her or her parents formally adopted you, and even then you'd only be step-brother/sister).

She needs to speak to a lawyer about legally designating you or someone else as her child's guardian should she die or become incapacitated. Once that's done and signed off on, she has nothing to worry about.