L.T.
First, what a wonderful thing you are doing for your sister. From experience I know there are a lot of things to deal with at this time in her life, having to research a rare disorder is not one of the things you want to have to do...so good for you for supporting her and helping her find some answers.
Second, I use to run a non-profit that researched information for new parents with special needs children. Two of my most favorite sites/organizations who provide both information and support are CDO and Unique. CDO (Chromosome Disorder Outreach) is at http://www.chromodisorder.org/CDO/Resources/Library.aspx this address will take you directly to their library section where you can go to the 18q deletion section and find numerous articles and information. There is also a chromosome learning section on the site which will help you/her understand the basics of genetics and chromosome abnormalities.
Unique is based out of England but they have local/regional contacts all over the US (I happen to be the NW Regional contact). Their website is www.rarechromo.org and they are a great resource for support and information. I have always found that they are good for letting me know I am not alone...which is so important.
Please keep in mind, no matter what you read on ANY chromosome disorder...every child is different and so are the symptoms and development that go with them. Try to stay positive (which can be hard to do when reading all the negatives about chromosome disorders). These kids are resiliant and strong no matter what their ailment. My son is a 1p36 Duplication baby...he was never expected to survive...he is 7 now and thriving...fairly typical even...and almost nothing like the other 1p36 Dup children on record.
Feel free to drop me a note if you want...as I mentioned, I had a nonprofit that actually did just this, found information, support, and resources for parents just like your sister...I am happy to help if I can. Many Many warm thoughts to you and your families!