I’m 13 years old and I have nieces and nephews from age 0 to 10. which of my nieces am I allowed to hold? Also, am I allowed to touch my sister (age 16)?

You should refrain from holding nieces from the age of 3 (you can hold her hand to cross the street or to stop her from running into the street ch”v).

You may touch your sister but the shulchan aruch writes that it is inappropriate to hug and kiss her.

See here: