Oleandrin is a single compound, and is considered the primary cardiac glycoside among several contained in oleander.  Here is the best information I have regarding indications that other compounds within oleander help transport oleandrin across the blood-brain barrier:

http://www.ncbi.nlm.nih.gov/pubmed/12416031?ordinalpos=2&itool=EntrezSyst...

You may also find some helpful information at:

http://www.tbyil.com/Oleander_Case_Reports_Studies.htm

I would suggest that you may want to look at possible faster acting initial therapies for someone who has only weeks left.  Oleander typically takes time to work.  It would be a good idea in my opinion, but perhaps in conjuction with something like Cesium Chloride or ozone therapy.  I would also suggest plentiful curcumin, such as the highly bioavailable Super Bio-Curcumin from Life Extension Foundation.  In addition to being a powerful cancer fighter, curcumin is also a good natural anti-inflammatory, an important consideration when dealing with brain cancers.