No, there's not much you can do really. 2 isp connections won't work on one phone line, you'd need a seperate one.

And if your sister's laptop interferes that much with the x-box connection then there's 4 things to consider :

1) Upgrade your internet connection speed

2) Ban your sister from using the net, or tell the router to only allow her minimal bandwith

3) Get another phone line installed and a seperate ISP

4) Flog your xbox 360 and buy a playstation 3