I think these first few days will be burned into your sis's memory forever. I know that is the case for me. It was extremely difficult to even believe this was really happening. I felt very sad for quite a while. I tried as hard as I could but would often end up in tears in public. At the same time, parents/caregivers must go into immediate action mode. Early intervention is key. There is plenty of debate as to whether that is ABA, DAN, TEACCH, chealtion, osteopaths, you name it (another thing to sort out), but I have never heard anyone say that early intervention is not the most important thing. While you sister is grieving, she imay feel that she is expected to become an autism expert and special education attorney. It's hard. It takes it's toll.
IMO, if your sister hasn't already she needs to contact the public school system (assuming she will use public schools). If the nephew is not in K yet, she shouldn't think that she has to wait until fall to call. The sooner she can start the whole sp. ed. process the better. It takes a while to get through all the meetings and evals. before an IEP (Individulaized Education Plan) can be implemented.
Next, from my experience, the most relief I felt during those first few months was when I realized that there were people that would help me and I let myself accept their help. This goes beyond relatives. Your state and/or county may have programs and there are non-profit organizations, as well. (Feel free to PM me, if you want some help finding them.)
As a pp wrote, truly what your sis is grieving for is the expectations and dreams SHE had for her son. This is still hard. I would never say it isn't. But, her son is the same today as he was yesterday and the day before and the day before...his life likely won't be like she thought/hoped. Her life won't be like she anticipated either. However, I think moms of special needs kids come to be some of the strongest women in this world. When she realizes that she is mourning (and IMO it is mourning) a dream and not her child, things will get better. This took a long time for me, personally.
Now, my son is 8. He was officially dx'd 4 years ago. My whole outlook on life has changed. I completely accept that he is autistic. I think the world that neurotypicals live in and expect him to live in and conform to is very difficult but he has great gifts that are unlocking with time. He experiences the world differently. I have been fortunate to communicate with quite a few autistic adults (some HFA, some Aspergers). The vast majority embrace their disability despite the challanges. Plus, I have been so incredibly impressed by how fantastic these folks are: smart, talented, creative (yes, creative). I want to give my son the tools to make his life less challanging, but I completely accept that he has autism. Now, I know it really is OK. It's a tough life for our whole family, but there is a purpose for everything.
Give your sister a big hug.

I will pray for her. She is hurting now, but she will be stronger later.
One last thing (yes, I must add to this long post)
I think a great book for parents just learning of a dx is
Children with Autism: A Parent's Guide, Edited by Michael Powers with a foreward by Temple Grandin. A lot of good info. to get started, but the best part is that each section has a few pages of comments from parents. The comments are often very diverse, but it made me feel like someone else knew what I was going through.
Oh, my son's neuropsychologist talked about the age 6 thing to us a couple of years ago. Sorry I don't remember the details, but it certainly doesn't mean that the child won't progress. My understanding is that typically there isn't going to be a jump down from high to moderate functioning, moderate to low. Typically, there won't be NEW autistic characteristics that develop, that sort of thing. Of course, this isn't 100% true. Plus, other factors could cause for a less desired change. It has nothing to do with the child being able to make progress. I guess, my interpretation now, NOT that words of DS's doc, is that at age 6 one can determine about "as bad" as it is going to get, uphill from there. I don't know if I completely agree with this though, but maybe the majority of cases
