My school grammar book (Ars Graeca) says that epi + genitive is a Genitivus loci and that this is derived from the IE ablative. I'm afraid it doesn't go into much detail on that, it just states that it's from the ablative
It says that these functions of the Genitive are derived from the IE Genitive:
Genitivus temporis - loci
Genitivus possessoris, subiectivus and objectivus
Genitivus quantitatis (partitivus)
And from the Ablative it has these functions:
Genitivus loci (by geographical locations)
+ Genitive with verbs (remember, see, feel, think... those sort of verbs)
it doesn't mention the IE dative at all, but loci is the first thing they mention when talking about the IE ablative.
ep' oikou doesn't sound much like an ablative, so I'm confused too. All the other praepositions like para and pros are motion away from something when with the genitve
. epi definitely has this motion to bit in it though epigignesthai for example... hmmm...
sorry i can't really help, all i could do is guess