OP, @Meadowfinch is completely right!!! Until you start setting boundaries their selfish behaviour will continue.
If I was you the first thing gone would be WI-FI & then wait. As soon as one comes to you get him/her to call the other sibling.
Tell them both to sit down & this is where you need to start with a tidy clean house is a non-negotiable in this house. Make it clear that the both of them are to make a start on cleaning it up until then you are not interested in any of their wants.
Tell them that you have watched them turn the house into a complete pigsty whilst you have been suffering with your back etc.
Make sure you tell them exactly what you have told us as then they can’t complain they never knew what you were going through.
If I were you the minute they get home WI-FI IS OFF! Then you have that conversation as they will have plenty of time to tidy up. They are adults so treat them as such.
As an example my children have been brought up knowing every action has a consequence. If they make a mess they know they will be clearing it up. My eldest will cook & my youngest much prefers cleaning/tidying up. The only thing with them 2 is that they can bicker & it can be deafening. Again I don’t let that slide as it’s unacceptable for them to be talking to eachother like that. They are adults in their early 20s and can still act like they are 12 but they would never not pick up after themselves as I stopped doing that when they turned 11. They have to respect the home that they live in and the people that live there.
This won’t be easy but they need to learn that this is not how you live & behave without a thought for the next person.
Why is your daughter not going to school? Is she being bullied? What is she doing all day every day? Tell her you will be ringing her school & setting up a mtg with her form teacher! She needs to get back to school.
If it’s a case where she can’t be bothered then it is time to end her phone contract. Kids these days have to learn the hard way I’m afraid & that things won’t just land in their lap.
This will be hard but you have to be the one who lays down the law right now as you are Mum & Dad rolled into one.
Also the Bank of Mum is officially closed until further notice and it will all depend on what they do & say now.
Oh & if your son does get a little lairy do remind him rent is not cheap outside of these 4 walls so he better decide whether he wishes to contribute to the household & clean up here or whether he chooses to move into his own place but the way he has been choosing to live will not continue from today.
You can do this, don’t second guess yourself and stay calm do not get angry.
This message needs to be relayed in a calm manner but sternly they need to see that you mean it.
Do not get into a back & forth.
You can do this & your kids need you to do this as this is no way to live.
Good Luck 🤞🏼
P.S I am here if you need to rant. X