Wizard's Holiday, by Diane Duane
Number 7 in the Young Wizards series by Diane Duane...
I read the first six books in this series quite a while ago and it was a lovely little surprise to find two more.
In this one Nita and Kit go on a sort of wizard foreign exchange program which is supposed to be a bit of a holiday for them and of course isn't. They have to help an ancient race of people from becoming completely stagnant.
And Nita's sister is at home and there are wizards staying with them as part of the program and while they're there they discover that there is something seriously wrong with the sun and they have to fix it.