I don't remember my parents ever trying to stop my obsessions, but other people I know have. There have been several instances where my peers have the mentality of "let's see how long we can get Namiko to shut up about LotR/Star Trek/chess/whatever other obsession there is" and they have actually said this aloud at times.
The best case scenario is when you can take your obsession and use it for something good. For example, I am obsessed with infectious diseases, so I took a class on diseases and epidemiology a few years ago. I was able to really get excited about what we were learning in class (which is rare). It might also be helpful if one can learn to force an obsession on oneself and so it can be used for something productive.
As for the specific obsession of graveyards, perhaps explaining to him that it is not a good idea to talk about graveyards in front of his half-brothers is a good idea. If he starts to, pre-arrange a signal (something like tapping him on the shoulder) or invite him into the kitchen to get snacks for everyone and gently remind him.
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Itaque incipet.
All that glitters is not gold but at least it contains free electrons.