The money is part of his estate. Get bank a copy of the death certificate and ask them to freeze account pending probate.
The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change.
My condolences to you on the death of your father. To respond to your question, it depends what you mean when you say your sister is "on" your father's checking account. You need to determine how the account was titled. If your sister was signing on the account under a Power of Attorney, the funds do not necessarily belong to her. If it is a POD (pay on death) account, your sister is entitled to the funds as the POD beneficiary. If the account is titled as joint tenants with rights of survivorship, your sister would be entitled to the funds as the surviving owner. It is also important to determine whether your father had a Last Will and Testament. You should consult with a probate attorney to protect your interests as a possible beneficiary of your father's estate.
I'm sorry about your father's death. Unfortunately, if this was a "joint" account, your sister has the legal right to keep all of the money in it unless there was a written agreement that acknowledges it was held in joint tenancy "for convenience only".
This highlights one of the reasons why joint tenancy is usually a poor solution in estate planning situations.
The information you obtain at this site is not, nor is it intended to be, legal advice. You should consult an attorney for individual advice regarding your own situation.
The answer depends upon where your father was a resident. If he was a Florida resident and the account is subject to Florida law, then section 655.79, Florida Statutes, might apply. This section creates the presumption that an account held in two or more names passes to the survivor(s) upon the death of one owner. There are exceptions to this rule and the rule is very fact-specific - one fact might change the answer. Because the bank might not freeze the account based solely upon your request, you should consult a licensed Florida attorney as soon as possible. It is probably necessary to obtain a court order if you seek to prevent the distribution of the funds.