[mdjlucan]

My brother says this is a plum tree, but what is this?

[hasbeans]

Looks like Plum Pocket.  A nasty fungus, the misshapen fruit need binning, preferably before they get furry and release more spores.  My trees have it bad this year.

[mdjlucan]

What causes this?

[Subversive_plot]

What causes this?

As hasbeans said, a fungus.  For more information, see: https://extension.umn.edu/plant-diseases/plum-pockets

[hasbeans]

Pruning in winter doesn't help, but sometimes it's just bad luck.  I've had Plum trees for about 10 years, 2 of those years I lost most fruit to Pocket and another two years to Moth.  The other 6 years I had too many plums.