He's also got a massive entitlement complex, he's so protective of Claire that he's almost stifling (trying to take the role of the traditional father who locks a daughter away from potential suitors), and he has no understanding of boundaries.
Eh, I think that's kind of harsh and a bit unfair. From his response at Emily's party when Claire tells him that she told Marten about being trans, his immediate reaction was "He wasn't a JERK about it, was he?" suggesting that Claire has frequently been the target of bullies. Claire didn't start off her arc having high-flying self esteem, so this may have been an issue. Also, their father fucked over their mother by cheating. That likely pushed Clinton into overdrive, not wanting another male to hurt the women in his life.
Also, Clinton's not, IMO, acted in the way you suggest. He didn't even pitch a fit when Claire told him that she and Marten were dating. He was mildly snarky about it. I actually thought Claire's "can't you be happy for me?!" was a bit overwrought there. It's not like he threatened to castrate Marten or suggesting pistols at dawn or somesuch.
I sort of feel about Clinton what I've felt about Dora. A couple of mistakes and anything they do is viewed through a cynical lens. No one is disputing that Clinton was extremely creepy toward Hannelore. But it's been ages since then and he's acted like less of a disaster in subsequent meetings. He loves his sister. He's not keeping her in shackles or guilt-tripping her for having a life. He's a 21 year old who has had to deal with a lot of shit - losing his hand, losing his father, and adjusting to having a sister. All things considered, he's not such a bad egg, in my view.