The old trust may still work, as long as the current assets are transferred to it--this will avoid probate. However, if there are a lot of changes like trustees and beneficiaries, you can keep the old trust name/date but "restate" it--it's an amendment that completely replaces the old trust.
For creditors, your father's can make claims against the trust, though they only have one year to do so after his death, and if creditors of you and your sister are a concern, I'd recommend keeping your distributions in trust and have a spendthrift provision. Money inside the trust is safe from the beneficiaries' creditors.
I've found the CEB (Continuing Education of the Bar) books on trusts and wills to be very helpful, and I'd recommend new powers of attorney for property and health care, since these have likely changed a lot in 20 years.
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As a licensed attorney you know the rules regarding practicing in areas of the law in which you lack competency or proficiency. You should either take some more CLE on the topic or associate with an attorney who has the requisite skill and knowledge.
Attorney Johnson provides some good information, but Attorney Davis is correct. God-forbid, if you screw this up, your sister will likely have something to say about it and all heck may break loose in your family and, potentially, with your state bar. Save yourself the pain by having your father retain an experienced estate planning lawyer. Good luck to you.
This information is presented as a public service. It should not be construed to be formal legal advice nor considered to be the formation of a lawyer/client relationship. I am licensed in Connecticut and New York and my answers are based upon the law in those jurisdictions. My answer to any specific question would likely be different if I were to review a client's file and have the opportunity to interview the client. Accordingly, I strongly urge you to retain an attorney in your jurisdiction with respect to any legal matter.
Don't do it yourself, it will save you time and grief and potentially alot of money by bringing in an experienced estate attorney. There are many minefields in estate planning that you may not see coming.