Surely it's not illegal to do the cable run on your Sisters property (presuming she owns it or has permission), it's just not legal to touch their equipment. Therefore it really depends who the blue pipe belongs to if it's not your sisters then obviously you can't use it but it doesn't stop you putting your own in.
The problem is even if you did legally do the cabling up to the fence it doesn't mean Virgin would use it, I have a feeling they wouldn't but just a feeling no experience with them myself.
My dad had a similar issue when Cable was first intalled as he was the first house on an unadopted road that wasn't cabled whereas the main road was.
My dad cabled it from his house right up to the join between adopted and unadopted road and asked them to connect which they did but this was years ago when cable was first installed and it wasn't even NTL then let alone Virgin Media. Took us a while to dig as the house to road was alot further than 5 metres unfortunately!
If you do go this route be sure to have enough cable left over for them to connect