First of all i need some context here.
Is it that your sisters boyfriend said He would pay half the cost for one for you and all you would have to do is pay the other half?. So he would have his TV and you would have yours?.
Or Is it that he wants you to pay half to use the one he already owns?. If so i dont see the point in paying half for the one he already owns, as that would just end up making problems imo if you or him or you sister etc wants to all use it at the same time who has the most say?. Id just continue using it as is and save up for your own one.
But if he is willing to pay half for one for you then why not?. If you have the other half you might as well get a deal.