Right, so there's your solution - when your sister takes the girls away for the week, you and Mick go away somewhere lovely, just the two of you. Doesn't have to be exotic, for example bring the car to Ireland, lots to see and do, some nice hotels, potter around doing the sights, enjoy good food, and just have a whole week to yourselves doing what
you want. Then put aside a decent sum towards a family holiday abroad next year.
On the subject of passports, I know the initial layout seems a lot, but do they last for 10 years over there as they do here? Did you say £275? That's only £27.50 a year and getting them now would cover the girls for school trips until they are finished school. I would never be without a valid passport because you never know when you might need it

Also, do not be waiting until the girls have upped and left to do things - none of us knows what lies ahead.