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.'