Relative*
I think a lot of stories about DID that want to be sensitive to the issue are bound to feel similar, as people want to do research and include the details that make it DID-ish.
I think a psychiatrist being a relative is an easy way to keep that knowledge close and have them be easily accessible in a story.
I could say Kill me heal me is just a copy of several DID novels about romance, but it's not. They just have similar alter roles present, child alters and opposite gender alters being relatively common in narratives (seeing as there isn't all that many DID stories in the media)
This is totally like the korean drama kill me heal me. I thought it just has slightly same personalities with the drama but then the girl personality appear... The difference is just this is yaoi. Even in the drama the female mc is psychiatrist and this one had a brother being one instead. Oh what a coincidence! :/