Okay...I advise not asking them every week 'any children yet?' It can take anywhere from a few days to two years or more to find your child, and then you have to go to meetings, panel etc before you can meet them
I wouldn't mention anything to your DC's yet. When you find out they are soon to be meeting new niece/nephew, then you could tell them they are getting a new cousin. I would simply tell them that 'x needed a mummy and a daddy because x's birth parents couldn't look after x'. I didn't tell my relatives much more detail than that because it's my childrens private story. Please don't ask them for details of their childs life, because it is private
When your niece/nephew actually moves in, then is the time to offer any practical help they need e.g. ironing, or walking the dog, as it's pretty overwhelming at first, and things like that tend to go a bit out of the window. I'm sure they'd appreciate practical advice like 'such and such pushchair and carseat was great for me', but stay away from parenting advice, because being an adoptive parent is totally different from being a birth parent, you have to use different parenting techniques quite a lot of the time
You sound like a very caring SIL yourself, just offer to be a listening ear if they need one, a practical help if you are able to do that, and celebrate when they become parents (some of my relatives ignored me when DD1 moved in because she wasn't a cute baby, and I didn't give birth to her :( )