I did notice the word Teenage.
Having 2 Lads who`ve just become non Teenagers & a Niece who`s 17, I can speak from experience.
The likelyhood is that your Sisters will almost certainly begin using Limewire, Bearshare or similar once on Broadband..... Its pretty much inevitable... So Bandwidth might come into play???
I`d recomend an Entanet Reseller, but you`re after not paying an Activation Fee, then you`re almost certainly into 12 months or more of a contract....
I`d personally recommend not entering into anything longer than 3 months, preferably 1 month contract.
As mentioned above Sky may be a good option, if your Mum already has Sky.... apart from the rubbish CS.