As an adoptive Mom, and one who has been trying, unsuccessfully, to get pregnant for four years, here's some advice: a Baby Shower is held when there is a child involved. That being said, her reasons for wanting one right away are valid. So, what if you throw her an Adoption Readiness Party (see Kaye S below) instead? If she needs a couple of things, and she knows what age she is looking to adopt, that will help. If that is not feasible, maybe family members who are no longer using cribs/car seats/changing tables could loan them to her.
A home study is mostly to ensure that the home is a safe environment, that there are not things at child/baby level that could be a danger, and that the couple is in a stable place in their own relationship, has a steady income, and are good people who can be trusted with children. It does help, but is not necessary, to show that you are prepared for a child by having a room selected for the nursery and, if not emptied/readied, at least in the process of being prepared. The inspector will want to see that you are serious about this decision.
When we had our home inspection, it was late in the process– we already knew the mother who was about to give birth (she had asked us if we wanted to adopt her coming child, so we got into everything a little late) and we were just waiting for the arrival of the little boy for the adoption process to continue. So we emptied out our computer room/office, and my Mom was generous enough to donate a beautiful bassinet, an extra dresser she no longer used, a changing table, and the same rocking chair she used to rock me and all of my 6 younger siblings in. So when the inspector came we had the nursery pretty much set; this was a surprise to the inspector. A huge one. She said most people don't have a room ready, and barely have anything. In most cases, a person is authorized/certified/approved for adoption before they even are allowed to find a child, and once they are approved, THEN they go about looking for a child to adopt. Which means that the inspector doesn't expect you to be ready for a child to arrive tomorrow.
As for an official Baby Shower, it should happen after she has been certified, and when she is actually about to adopt a child. Go ahead and throw her a shower then (for one thing, you know she's getting a child, and you'll– hopefully– know the age/gender of the child)! Celebrate her coming into Motherhood! Celebrate the arrival of a dearly desired little one!
So, how to handle it? Say something like this: "I think it would be hard to throw a shower since we don't know how old of a child to shop for, or whether they will be a boy or a girl, but if you need some things to get ready I would be more than happy to help you get them! How about we have an Adoption Readiness Party, instead? And when you find the child you want to adopt, and it's a sure thing– you've been approved, and the court date is set– of COURSE I will throw you a Shower! I would be more than happy to! It won't be weird; a shower is celebrating the arrival of a precious little one into the family, and celebrating your transition into the role of Mom!"
Telling her this way will let her know, subconsciously, that it is something to wait for until a child is actually in the picture, and will also reassure her and warm her heart that you will be happy to do this for her, and that you are ready and willing to look for ways to help her now. She will not miss out on this important celebration, and you are working on her side to make her dream a reality.
Having the shower AFTER the arrival of the child is also okay. I was given two showers, one by my mother, and one by my sister-in-law, after the adoption of our little baby boy (he was only 6 days old when we brought him home). I cannot begin to express how much they meant to me!!! It was okay, too, having our son there; the ladies LOVED holding him and talking to him and passing him around. Our son enjoyed the attention, I enjoyed the honor of being recognized as a Mother (and him being recognized as my son), and everyone was thrilled and happy.
For an Adoption Readiness Party, some gifts she could get that don't have to be gender/age specific (it sounds like she wants to adopt young, anyway): adorable baby blankets, bibs, bottles, pacifiers, wipes, stuffed animals (puppies and teddy bears are a safe bet), and keepsake items: a Remembrance Pillow where she can write/embroider the child's name, birthdate, name of parents (hers and hubby's), a memory album kit or beautiful photo album to store those precious first pictures, a Memories kit with a keepsake baby fork and spoon and two little containers for a First Curl (lock of hair) and First Tooth. Crib sheets and bedding are good.
Something good to suggest: that she have a Hope Chest, a place where she can save things as a 'hope for the future'. This will allow her to indulge (if she chooses) in baby items a little, while still allowing the distance of 'this may happen, and it may not.'
That's a big list, but it gives you and idea for some 'safe' items. More specific items (clothes, diapers, toys, anything gender specific) can be given at the official Baby Shower when the adoption is for sure.
I hope this helps! God bless, and I will pray for your Sister-In-Law!
M. D