Such fantasies are really normal la.
If u think with heart, of course all bros will tell u, "DUN DO IT IN OWN BACKYARD".
If the dickies thinking... actually, u may ask wifey if she ever fancy letting u do sis, and maybe together with her???
Nothing venture nothing gain. Maybe u can unravel a deep unknown secret and TIO BEH PIO!!!
- now that is my dick's 2 cents worth!