the GP prescribed this to my sister (GI troubles, vitamin deficit,...)

http://metagenics.eu/en/19/consumer/308/310/modules/products.phtml?id=1980

I'm wondering about the presence of bacterias and piperin, curcumin, and other stuff.

Will the bacterias remain alive with the compounds of this powder? I'm not a fan of these formulations, containing so much vitamins, they probably haven't any efficiency, but my question is really about the bacterias.

She is drinking kefir on a daily basis so I think this is not a problem even if the powder don't work.

You are right, the piperine as well as the curcumin from the curcuma extract and the polyphenols from the green tea extract will all kill the bacteria.

Another problem is that the oxalic acid in the green tea will bind the calcium in the formula.

One other issue I have with the listed ingredients is that they do not state the forms of nutrients other than the magnesium.  Is the B₁₂ the inactive cyanocobalamin or the active methylcobalamin?  Is the calcium the poorly absorbed calcium carbonate or a better absorbed acidified calcium.  What form of chromium, picolinate or the more effective polynicotinate? Etc.  Being that they did not list this information they likely went with the cheapest, least effective nutrients they could find.