An option for Leola would be to go with the story that her mother died...but have it be in a very narrow timeline. If you said that her mother died AFTER her name was Reaped for the 75th games, but BEFORE attending training, then that would probably make the most sense for why they decided to pass the torch on to Leola. Because if her mother had died after the games, logically speaking, there would still be enough Victors in District 1 to have a pool to choose from. District 1 takes their training very seriously, and have won several times if I recall correctly.