A: It depends on what the deceased person owned. If the person owned real estate or bank accounts *in their name only*, then probate would be necessary. If they owned real estate as "joint tenants with right of survivorship", probate would not be necessary to transfer the property. If their bank accounts have a payable on death provision, they do not need probate to transfer those funds. Also, currently in California, if the total value of the person's assets are less than $150,000, then a simplified probate procedure can be used.
A: The best thing you can do about it is to have your parents get a lawyer who would represent them (and would not represent you or your sister). The lawyer could help them understand what they have signed and help them make changes that your parents think are fair.
Of course, you or your sister might still think the result is not fair, but at least you wouldn't have to worry that your sister is being sneaky, bullying your parents, or cheating you or your parents.
A: If the savings account is in the name of more than 1 person, with "right of survivorship", it will go to the other person on the account. If not, it goes to the person (or people) named in the account holder's will. If there is no will, it goes to the next of kin. Who is next of kin will be determined by your state's intestacy laws.