Welcoe to the STD forum.
First, "I believe I have...an anal wart" doesn't mean much. You need professional assessment. There are plenty of causes of anal area bumps other than warts.
Second, the genital HPV types are not spread by hand to hand contact. Even if you do have an anal wart or other genital-area HPV infection, there is no risk of transmission to your neice or anyone else, except of course your sex partner(s). You should not mention your concern about HPV to your sister. That would only alarm her unnecessarily, and probably embarrass you. Just let it lie. But see a health care provider about the bump.
Regards-- HHH, MD
thank you very much for your insight