Disclaimer: Shingeki no Kyojin is not mine. If it were the Lekasa ship would have sailed long ago. LONG LIVE THE ACKERMANS, HUMANITY'S STRONGEST PAIR!
They'd agreed to keep it between themselves.
This wasn't done out of fear or even a concern for the feelings of others. It was to preserve their privacy, something they both deeply valued.
They planned to reveal the truth slowly, to ease their comrades into the situation.
It would be best to cause as little friction as possible.