If you haven't done so already, I would say this is something you should likely speak to an attorney about near you. Laws of survivorship and estate execution can get tricky without some clarification from a lawyer.
I am not an attorney, but I did a little research on your question. From what I've found, the funds in the joint investment account to which you had the right to survivorship belong to you and only you and are not part of your mother's estate (unless her will says otherwise). If you've already wrongfully given that money to your sister, that's even more of a reason to get an attorney involved to help insure you're returned what's rightfully yours. Good luck with this matter.
I hope think link from Cornell University Law School helps clarify the definition of "right of survivorship":
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