terakhir = last.
termuda = youngest.
It could be the same for you, I'm happy for you if that is the case, but it's certainly not the same for me.
That's only applicable if your younger brother/sister is still alive.
Open up your mind and imagine the following if you can:
Person 1: "Are you the last child?"
Person 2: "No I'm not, my parents had another child after I was born, but she doesn't live anymore."
"She was killed."
"For my parents, my sister was their last child."
"I'm their youngest child who is still alive, but I'm not their last child, that was my sister."
Thank you for your response and explanation. I guess it all depends on the context: Person 1: How many siblings do you have? Person 2: Three. Person 1: How old are you? Person 2: Twenty. Person 1: Are you the youngest child?