From the infinitive 'to sible' (pronounced 'sibble'), meaning to perform some form of sexual act with another persons
brother or
sister (preferably younger for greater novelty value...though not
TOO much younger). For clarity in use, the given example would mean that to sible 'her', would be to sleep with 'her' sibling.
'
Dude, I would totally sible her...soon as her younger
sister's street legal of course.'
'For sure
dude! I'd give her the sibling of a lifetime.'