At your own house

You are at home were you and your two sisters lives. You and one of your sister are in your 20s. But the younger one is a child. You were their caretaker. You always wearing baggy clothings.

Your parents has been missing before your second sister was born. The previous sentence doesn't seem to make sense, but there actually an explaination for it.

One day, you approached your little sister to tell her something. Your other sister was also here.

"Sis, there is something I need to tell you." you said

"What is it?" she answer

"Do you really know why our parents isn't here?"

"Yes, sis told that they went missing after I was born?"

"I only said that because I didn't want to give more details then." Your other sister said

"Wait, so mom might still be alive?" The younger one said

"Actually, to tell you the truth...

...

I am your mom." You said

You then undress in front of your kid sis, leaving only your underwear on. From what she can see, you have all of the female proportions from the neck down. Including child-bearing hips.

"So you're not my brother?"

"I'm still your brother from the neck up. But the rest of me is all female. This used to be mom's body."

"What happened to her?"

This time your other sister decide to answer.