I agree with you so much.
I would also like to add - If the dad was gay from the beginning (he knew and all) then he would also be to blame for marrying Ken's mother. However, if the dad didn't understand his sexuality completely or that he was bisexual, then I don't think he should be blamed for having a "fake"/"forced" marriage or whatever.
And honestly I don't completely blame the dad for not saying anything. With how prejudiced/how big of a grudge the two kids (Ken and big bro) had against their father, it's hard to say anything. I mean they probably wouldn't even listen/believe their father's words only. So the dad and mom should've made everything clear right at the beginning. But then they were kids at that time, so they didn't say anything and then this misunderstanding continue. Now, even if the father wants to clear the misunderstanding, he probably will need the mother to come and explain too since Ken probably won't believe him. Not to mention, they all got used to the current situation. The dad might also think the two are upset with him for being gay and not know the misunderstanding that Tarou was a home wrecker.
TL/DR: I'm not mad at Ken, even though he shouldn't have taken his anger out of Kirara. This is the Dad and Mom's faults for not having a very much needed conversation with their children about what happened. This is what happens when communication isn't there.
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I think I had this conversation with someone on here last month but see? From what Ken said, THAT I'd why the dad needs to buckle down and have a very long conversation with his children. He needs to be open and honest and explain things to them and then maybe ask them if they would be willing to have an open and honest conversation with Tarou-San. The Mom needs to be in this somewhere too.
Minus, Tarou, the adults should've at least talked to the children by now about their relationship and how it ended. Maybe they wouldn't have understood when it happened bc they were chosen, but now they are older and should be able to understand the world a little more to have this very serious conversation.
If the Dad and Tarou did have an affair and broke up the marriage, then that's a-whole-nother beast that needs to be discussed and in my opinion, the brothers have a right to be upset. BUT if the marriage was emotionally, mentally, and physically over--and they were just legally together but separated--then the situation might not have been so cut and dry and that is where a long conversation needs to be had.
I'm not mad at Ken. He's a ting naturally actually bc he's still too angry to process that relationship, but that's not his fault. It's not his fault, it's not Tarou's fault and it's certainly not Kirara's fault. It's the Dad and Mom. I do, however, feel that Ken shouldn't have been mad at Kirara bc he has nothing to do with this BUT.....stilll....I get it. Right now...he might feel betrayed. Maybe Kouichi understands things now and if he does, that's great, but people process things differently.