to expect my sister and her dh to pay back money they owe before they sod off on hols

You are OT being unreasonable.

My mum was in dire straights early in 2004, cause my gran and granddad both lived with her and my granddad had just died leaving hefty funeral costs and less contribution towards the bills. My mum got a very threatening final demand letter from capital one and owed nearly £3000. They were going to send in bailifs, the lot. I got upset cause I'd had all this early in my life when my dad and mum split up and mum was hard up cause she was studying to become a teacher. Anyway, my husband and I had some stocks and shares and had no mortgage on our house, so I begged him to lend her the money and she said she would pay back 100-200 a month, which would have seen it paid off within about 2 and a half years. No interest, nothing, just kindness!

Anyway, 3 and a half years later, she has paid back much less than half because she always has an excuse. She has, however, found the money to buy one of these timeshare things where you can stay in certain hotels for a few weeks a year; she buys nice clothes and jewellery etc. She goes on other holidays lots. Her teaching salary was good back then but she has now gone through the threshold and is on about 35000 a year. That and because my nan lives there, she has no car to pay for, discounted rates, nan buys the food, us 3 kids have left home now. It's ridiculous. I was a student back then and am about to become one again; I was very ill last year, but apparently her conscience just will not prevail. So I have a lot of sympathy. We could really do with that other 2k right now as we are having to remortgage to do some work to the house before having a family of our own. Some people, like my mum, will always be like this though.

good idea re: phone pulling out

our parents are dead though so i fear they wont help either, his parents live in the arse end of cornwall and are about 150.

YANBU.

Just unplug the phone so she has no way of getting in contact with you or leaving you that kind of message.

She can call your parents or his parents/siblings and get them to help out .

If they kick up a fuss on their return, you can always ask them to explain to your son why his money was not returned to him.

No you're not being unreasonable but I wouldn't lend them any money again if they're that careless about sorting out debts.

Of course you are not being unreasonable.
But you do need to tell them, calmly and clearly, that you need the money now, before they go on holiday.
With a couple of hours to go until their holiday, it sounds as if you have left it too late to have that calm, clear conversation.
If so, perhaps it's better to remind them of what they owe, and make it clear that you need repayment within a couple of days of their return.