The was no need to make Leia the sister of Luke even in the “lets get it finished fast” mood that Lucas was in at that point in his life.
Yoda could have been referring to Anakin as the ‘other’ hope.
Ben could be all negative about Vader and how he is beyond redemption and needs to be destroyed for the greater good and Yoda could be all wise and remind Luke there is always hope.
On his death bed Yoda reminds Luke of his earlier lesson about the nature of the light and dark sides. All that was needed was to have him reflect on how it might not necessarily be curtains for Anakin, that he might have been wrong and Luke might be onto something about redeeming his Father.