"They split and put all of their assets in each of our names before they died. They didn't want to do a will for whatever reason."
I don't know what the other assets are, but in regards to a bank account,
If the account is in the name of the parent with the sister as co-owner, that means the sister was an equal owner of the money with the parent, and is now the sole owner. This means the sister can access the money without going to court, but it also means that, since she is the sole owner, anything she shares is a gift from her to you.
As the recipient of a gift, you don't pay taxes on the gift.
As the donor/giver of a gift, your sister would have to report any gifts of more than $17,000 to the IRS on form 709, but no gift tax is actually paid unless her lifetime total of gifts is more than $13 million. (Large gifts must be reported so they can be counted against the lifetime limit.)
Likewise, if you ended up the sole owner of any of your parents's property, and you share it with your siblings, that would be a gift from you to them in the same way.
If there is a house involved, you and your siblings need to see an attorney or tax planner before you think about selling it. When parents give a house to their children, it can result in a large capital gains tax for the children when they sell, that can sometimes be avoided by proper planning and legal advice.
