If my parents are committing tax evasion, can they force me to pay their tax after recently turning 18? I’m only on PIP and cannot work or access an educational setting. (UK)

Tried posting on the LegalAdviceUK subreddit but it gets taken down immediately.

Disclaimer: I do not have tons of information due to my family being very against talking to me about literally anything.

Some context:

My stepdad is about £10,000 in debt from avoiding paying Child Maintanence. He stopped using his bank account about ten years ago, around the time those payments were meant to start. Loudly declared earlier today he stopped paying his taxes too. He is a plasterer and paid with cash only. Of course, he's telling them he's homeless, no income, no job, single, and doesn't live anywhere near where we live.

We received a letter a couple days ago saying he's now at risk of six weeks in prison and being suspended from having a driving license and passport for 2 years. We've been having letters come basically every week asking him to pay. Family is acting like this is an unforeseeable turn of events. In their own words: "Sending me to jail isn't going to get them the money, so what's the point of it?"

My mom, who supports and encourages his behaviour including him literally stealing my debit card on multiple occasions, is unemployed as she looks after my baby sister. She is on PIP and Universal Credit. She has been using my PIP solely for her benefit ever since, well, whenever she claimed it for me when I was younger. In recent weeks, I've finally managed to remove her as the appointee and get it paid into my account now, but it still funds the family car through the Motability scheme. I am hardly ever in the car. Hopefully, this will be sorted next month.

This is what is what I am worried about:

My mom is against me being spiteful and horrible by not wanting to pay for her car. She's said that if I end the Motability scheme, I must pay her £200 instead towards taxes. I don't trust that it'll be used for that and not whatever my mom is overdue to pay on Clearpay, usually like £600, or to my stepdad.

She's told me that because I have turned 18, the taxes have gone up as there's now another adult in the house. I assume she means Council Tax. However, I am confused. From what admittedly little I know, the tax only increases when it's moving from a one adult household to a two adult household, not a two adult to a three adult household.

Am I mistaken? If not, is there no, I don't know, law or anything that's on my side here? I'd assume not but it feels really unfair. If I truly must pay the tax, then is there a way to pay it to them directly as opposed through my mom?