Hi D.,
What a caring and consientious Aunt you are! You are wonderful to care so much about her health!
The best "brain" food supplement is of course a high quality, uncontaminated fish oil. Fish oil contains DHA and other essential omega-3 fatty acids, which are imperative for proper brain development and funtion. Kids who get adequate amounts clearly have higher IQ's. I use the Carlson's brand with my daughter. They have a plain fish oil and a cod liver oil with vitamin D. I use Cod liver oil in winter months (Oct-May) when sunlight exposure is limited and vitamin D production is down in the body. I switch back to the regular fish oil in summer (May-Oct) when we are outside and getting plenty of natural sunlight without sunscreen. We use sunscreen judiciously to prevent burn during the hottest times of the day, but make sure to have some safe unprotected sun exposure as well to insure vitamin D production, which is the safest way to get vitamin D so you don't overdose.
You can get these fish oils at many local health food stores (e.g. Roy's Natural Market at Preston and Royal, Royal Abrams Pharmacy at Royal and Abrams, Whole Foods). You can also perhaps get a better deal at www.mercola.com. He also has good articles on the benefits of fish oil, so you can educate your niece's mom, as well as general nutritional info.
Carlson's brand is one that I am comfortable does not contain harmful contaminants. This is a concern with fish oil, since the world fish population is entirely contaminated. Fish oil properties are also delicate and prone to deterioration, unless it is processed in a quality fashion. There are other brands out there, but I have confidence in Carlson's. Once you open it, it must be refrigerated to insure quality.
She may take the fish oil plain from a spoon, as it has a mild pleasant lemon flavor. Otherwise, perhaps you can mix it into apple sauce or mashed sweet potatoe or avacado. A breastfeeding mother should take a fish oil supplement to insure the baby gets sufficient amounts.
Other good resources:
William Sears' The Family Nutrition Book
Ruth Yaron's Super Baby Food
Good luck!