Although humans may not do it as well as other primates, we do still have a very effective fibre fermentation/digestion process going on in a healthy gut & in addition to that there are indications that it is quite healthy to have a good balance of fibre in the diet. Have a look at the following papers:
http://www.ajcn.org/content/31/6/927.full.pdf
Short-Chain Fatty Acids and Human Colonic Function: Roles of Resistant Starch and Nonstarch Polysaccharides
The first one, done in 1978, was analysis of fecal matter between controls and subjects who had lower intestine removed(illness) determining what % of fibre is digested.
The second one is a lot more detailed (36 pages) discussing all the different aspects of fibre digestion in humans, some of the key features being the a certain portion of fatty acids produced are absorbed directly by the gut to power it's own process and the heat generated from fermentation contributes significantly to thermogenesis, hence reducing the energy demand on the rest of the body.
That's not to say that being Vegan is healthy, just that eating Fruit & Vegetables is healthy for most of us, another incidental thing that I read recently and need to look into better is the presence of GAG's (GlucoAminoGlycans), these are complex carbs that form the bulk of connective tissue in the inter cellular spaces in skin, fat, muscle etc., well it appears that these meat "carbs" are also digested by the same fermentation process in the lower intestine, but I do need to look into this one a bit more. So whether it is plant fibre or animal fibre we still need to maintain a healthy gut flora for proper digestion.
Ribbons
Re your aunt, do what you think you should do, if you personally feel obliged to say something, then do so, speak to her, tell her of your concerns & why but do not harp on about it, say your piece then let it go. She will come to a point in the future and remember your words, if you keep going on about it, she will most likely dig in even deeper, people are like that they will "cut off their nose despite their face".