I have heard that there are opinions that hold there is an isur to touch a sister over the age of 3. Is there validation to that, and what is the halacha?
According to the Chofetz Chaim already a girl from the age of 3 is considered a woman as far as the issur of davening in front of her exposed body etc. The Rambam writes that one who hugs or kisses his sister is considered a simpleton. So putting the two together seems to be the source for the opinions you quoted. The Chazon Ish is more lenient and holds that she is only considered a women when she starts attracting attention.
Rambam Issurey Biyah 21:6, O:C 75 Biur Halachah ד”ה טפח, Chazon Ish O:CH 16:8