Does your sister in law have kids?
Now, just food for thought... not only will you be gone with your eldest child, BUT, Daddy will be gone too. So Mommy AND Daddy will both be gone that same week. Will your 5 year old and 20 month old adjust?
If your SIL is not used to handling kids, well, she may have a hard time... especially at bedtime. That is a doozy, even for seasoned parents.
Does your SIL know how to feed kids, for each age?
Or will you pack food for your kids, for each day & night?
Does your SIL know about how to put them to nap? And where?
Will she be babysitting at your home or hers? Is her home baby/child safe and child proofed?
Can your Nanny possibly do an extended stint for this time period? Day and night? Ask her... since your 20 month old is used to her.
Yes, your 20 month old will have her 5 yr. old sister with her, right? So at least that's good.
Yet, you promised your 9 year old this trip... and have been planning it for a long time. I"m sure she will be disappointed to have it canceled on her.
This is very hard.... I don't know what I would do myself. But I would go per my youngest child...and what my instincts were about that. In relation to the babysitting concerns...
Sorry, I don't have an answer, just some thoughts I would have in my own head about it.
All the best,
Susan