I got curious and re-read the chapter where they all go hiking together: My suspicions were true. Seichi is letting his personal grudges cloud his judgement (or the manipulation of his mother, for that matter). He claims they always favored Shigeru, but all Shigeru did was go and compliment their grandfather, humoring him and as a result getting one of the hiking sticks he had for the ocassion. Most likely, If Seichi had asked or done the same thing, his grandpa would have gave him one just as easily. Additionally, he seems to completely forget that his grandmother asked him if he was faring well during the hiking. If nobody gave a shit about him, this wouldnt have been the case. Seichi is biased.
Also, he says that while Shigeru got the "super duper hiking pole", "they" (as in his family in this context), gave him a shitty stick. And the truth is, Shigeru gives him one, clearly out of consideration. He asks Seichi to come along with him, helps him out and gives him the stick to help him during the hike. Is interesting to note that before that, Shigeru seemly is aware of what his mother thinks of Seichi's situation and his relationship with his mother, which leads to him bringing this up to Seichi but backing right up when he sees he is not getting the response he was expecting (Seichi gets defensive instead of thinking about it for a minute).
While Seiko stated she was never happy with the idea of living a family life (as in, having "strangers" come into her home all the time and her having to put on a good housewife act when she didnt want to), and how these tensions started to build up to the point she felt a grudge so big she pushed her nephew off a cliff (srly wtf), It also makes me wonder how much of Shigeru's actions were influenced by his own mom. Did he act the way he did towards Seichi, as a way to try and have him realize he needed to be more independent from his mom? Did he do it because in his own household that was deemed acceptable or his mom encouraged his behavior??? Questions, so many questions.