I'm going to assume you are using fresh grapefruit. Not as easy as the slices you find in a jar, but much more flavorful. If using fresh grapefruit, peel them, remove the pith, and section and be sure to work over a bowl so you can reserve any juices. If you use the jarred slices, use the liquid they come in.
It would help if you had a cup of liquid, so in a medium-sized saucepan, add enough water to the natural juices to end up with a cup.
To this, add the marmalade and sugar and bring to a boil over medium-high heat. Be sure to stir until the sugar dissolves.
Add the fresh cranberries and cook until the skins pop. This should take about 5 minutes but can take more. Immediately remove the pan from the heat and let it cool.
Add your grapefruit slices, cover them, and chill in the refrigerator.
Before serving, slice the bananas, add them to the fruit bowl, and serve.
