You may want to correct your post above, because it is unclear whether it is an uncapped or capped account that you are referring to.
Usually capped accounts allow up to 4 simultaneous connections, where as uncapped accounts allow only 1.
You can simply give you ADSL account's details to your other family member. I'm running 1 WebAfrica account on 2 separate ADSL lines without any issues.
The prerequisites are that your other family member has to have an active ADSL line + that your ADSL account has to allow 2 or more concurrent connections, UNLESS you're not even dialing that ISP account from your ADSL modem any more.
Please accept my apologies for the vageness in the original post. It is an uncapped account and I am not using it at the moment with the possibility of not using it in the next 3 months.
Usually ISPs lock your uncapped account to the first telephone number it was used on. You might have to check with your ISP that no such lock is in place or add the other number onto the white list.