He could have also just said that his sister worked at a maid cafe for some time, not even implying his own visits.
Also the needed help was very expected, though I guessed Tsuruko would be asked to help out, but that might still happen.
On another note, I don't get why she wouldn't be a maid. It is the perfect alibi for any maid stuff or photo found at home by her parents: "Oh, that#s from the cultural festival, remeber?" Way too good of an opportunity to be missed. All these characters fucking suck at coming up with responses on the spot, not to even speak of lying to save themselves -.-