How do you protect yourself from a family member trying to gain conservatorship over me?

In most states the court will appoint a lawyer to represent you. Even if yours does not, showing that you are able to manage your own finances should defeat your sister's claim that you need a conservator.  If you are having trouble (and even if you are not), appoint someone you trust as your agent under a Durable Power of Attorney.  A court will usualy not appoint a conservator is there is a Durable Power of Attorney in place.