Adam stated "took my mother away", which (strictly speaking) is very vague. That could mean she was killed, kidnapped, stolen, imprisoned, anything.
In terms of the booklet source we have, Maria was stated to have established the Evillious Calendar. There's also no apparent reasoning for the Senate to do the afformentioned killing/kidnapping/imprisoning so what Adam refers to is hard to say.
Since it's a minor detail (and we honestly have no idea what happened to the Queen), I'd ignore it and wait for another light novel/song/album booklet to elaborate on it.