Hi, I don't post that often, but to answer your questions.
Firstly do not rely on online colour vision tests as the representation on your monitor is not accurate. There are a couple of professionaly available programmes and what we have to do to calibrate them is lengthy. One of the other problems that the test you linked to is that it relies on colour naming - a not very reliable procedure, except in forced choice tests eg lantern tests where you only have a choice of primary colours as opposed to hues.
Regarding Ishihara (and other pseudoisochromatic tests) they should be done using the correct illumination ie daylight or standard illuminant C. But be aware, and I have mentioned this earlier in the thread, the results with Ishihara can vary from edition to edition and also printer.This is due to ink variations as Ishihara never actually specified them using say the Maunsell system. More reliable are the likes of the HRR test and the City University Test as these use specific Maunsell colour chips.
The note about illumination applies to the colour matching tests like the Farnsworth-Maunsell 100 Hue test or the Farnsworth D15.
As to the prevalence about 10% of all males have some form of colour deficiency and it is X-chromosome linked which explains your brother.
Certainly try a lantern test, you may pass it because of the reasons above, Ishihara can be give false positives as well as shown up the mildly colour anomalous.