I’m a parent of adult kids, including a ND adult. I have answers for you, so you need to wonder no more. But yet, you will keep posting no doubt.
You were an adult (in late 20’s or 30’s I think?) when YOU chose to get married, and YOU chose to have a child in an unsuitable situation, and then seemingly did so again. Given this, your parents have ZERO obligation. That’s the first thing you need to process. You have not processed that.
I would think your parents HAVE given you answers though. I highly suspect you either can’t process or compute these answers though, so in your mind they have not been given. A bit like Harry’s ‘truth’ as opposed to reality in a way. Your father likely mused on the possibility of selling his car to assist, and then in working that through that rationally, realised that it was like stopping a dyke leak with his finger and realised it was pointless and wouldn’t change the ultimate outcome. I suspect this was conveyed but as it wasn’t aligned with your ‘wants’ you refused to take it in. I suspect many explanations have been given that you refuse to take in, which equates to your understanding of no one explaining.
You seem to think your sister is favoured. I don’t think ‘favoured’ is correct. It’s probably more correct to say your sister has not given your parents the same level of grief. She likely didn’t get married when it was unsuitable, pregnant (twice) when it was unsuitable, and hadn’t need to be bailed out constantly as an adult. That doesn’t mean she is favoured, just acknowledged as not being as problematic as yourself.
Re your in-laws, I suspect they knew exactly what shit-show was occurring and wanted to ‘save’ their grandchild. Not only from yourself, but also their own son. They didn’t destroy your marriage as such. They saved an innocent child who was not being looked after suitably (a court has agreed with this seemingly). If your marriage was a casualty of this, so be it. The important thing was the child. Not you. Not your marriage. It’s not about you at all at this point. You can’t understand this though. You refuse to understand.
As to your parents going on holiday, just stop. Of course parents with a child in their late 20’s/30’s/40’s should go on holidays. They don’t need to plan their lives around yours falling apart because of the choices YOU made. Don’t be so ridiculous. Their holidays ARE important, if you are like this as an adult then I imagine it was a lot worse when they DID have an obligation to you as a dependant so god knows they deserve holiday relief with you as an adult.
Then, you NEED to live at home (to support yourself seemingly, and retain custody of your second child). Your parents facilitate this, but don’t have to. If they are narcissistic, physically abusive, or just not to your liking - then move the fuck out, and this ceases to be an issue. The end.
There really is no more or less to it than that.