Hayfrm,
I am sure that was .046% not 4.6%. A DUI in the states is .08%. Death commonly occurs at .4%, and a BAC of >1% has never been recorded on a subject in the USA.
A .046% an be had by drinking 3 or 4 beers in a couple of hours, and of course any alcoholic can do that with their eyes closed. Or it could be a residual BAC from a hard drinking bout the night before. In any case your MIL either is ignoring the facts or had the wool pulled over her eyes. Don't count that out, us As are experts at it. In any case, your MIL may not be the best choice to look after her after detox.
Best of luck in whatever you decide to do.