When Eren was disguised as a wounded soldier in Marley to make it more believable he started a family with a woman that was already sick and dying so nobody said anything

Soon Eren and the one had You

Then a couple years later your mother passed away and when everything went down Eren make the decision to take you with him on the airship

Now Eren was back at Paradise and now sitting in a room holding you

Mikasa: was staring in shock

Armin: he was also staring in shock but try to compose himself to ask "E-Eren when did you...Get...."