On the one hand, it's definitely a terrible thing the father did to pit them against one another. The only excuse I can think of is that the author wants to establish that this is "just how things go."
But, on the other hand, looking back at the very chapter this is brought up. I feel it's really more of his father explaining the reality of what could happen if he doesn't get his own act together and prove himself. That whole chapter made it clear it was Kurt's POV. Kurt has been shown to see things the way he WANTS to see things. Also in that chapter showed that Kurt was worried about ALL his brothers potentially taking his position as he goes through the list. It wouldn't be a stretch to think that he distorted his father's words.
You could also take it another way for the MC. If the MC stays, he'll have nothing. Eighth son of a piss poor family and no prospects of inheriting land, short of ALL of his older siblings dying, which is a terrifying thought in itself. So, it would be better to "chase" him out.
IIRC, while doesn't show it in the manga, but from the MC's POV, even before he started to read and learn about magic, he didn't have good relationship with Kurt. Which makes it all the more reasonable for their father to say something to Kurt after seeing the MC excel at magic.
@Lord-Raine, the part where Kurt tried to be "reasonable" appears less about being a decent family and more about using the MC for his own benefit, which is clearly what their father says... you want to use him, but that will only backfire. And it's because he hasn't shown himself a capable lord.
This is pretty long winded, but the short of it is, I think Kurt distorted what his father said and is only making excuses for himself, as usual.