At 18 you won't yet have the self assurance needed to deal with people like this very well without support and guidance.
Where are YOUR parents when they visit?
I could easily be YOUR mother (my dd is 17 and I was bloody ancient compared to you when I had her!) and if my dd was in your situation I wouldn't be leaving her to deal with nightmare "in laws" unless she was very insistent (ie I knew she meant it). So I could be there to defend her corner if necessary and if they pulled the kinda crap you're describing I'd be marching them out and telling them not to even think of asking for another visit without a genuine apology and a change of attitude!
If your mother has offered such support TAKE IT! It is not a weakness on your part to need such support. If she hasn't ask her! I'd absolutely not have a problem with dd doing so and be happy to help.
I suspect it's very much a case of your youth being taken advantage of.
Frankly I'd also be having strong words about why THEY weren't having several strong words with their son who's abandoned his responsibilities!
Please tell me you've at least applied for maintenance?
"One thing I have learned with age is that you can say anything to anyone if you say it politely, firmly and calmly." AbsoBLOODYlutely! Amazing what you can get away with saying with the right tone and head tilt 😉
"I don't understand all these posts about grandparents rights...
In previous threads any threat of this in media has been swiftly closed down as never workable." Esther rantzen is leading a campaign and there's been a few recent court cases go in the grandparents favour. There's a couple of mners currently having difficulties due to this.