Hmm, have you considered a middle ground? The choice of killing the siblings seems very interesting, and I think it should have its own worth. I’d recommend considering these questions:
Does it make some choices easier in the future, for example about succession, o gaining things, simply because there’s no competition? On the other hand, does it make anything harder? Would killing any of these characters bring in, for example, their mother as an antagonist instead? That seems like a good way to get yourself invested in the lack of the siblings, and not to feel so invested in their presence that it risks railroading the story just to keep them.
Besides, having a choice affect the game this heavily enhances replayability, provided that taking away the characters doesn’t simply… take away a part of the plot. As in, is the game and story lesser for their lack?
Also I don’t think it’s bad to be into the siblings, I’d say just make sure the overall narrative doesn’t suffer from it.