"I am the oldest child, she is the youngest child."
Translation:Saya adalah anak tertua, dia anak termuda.
The odd thing is that I don't think I've seen similar statements phrased this way. Sure, you can say it like this, but since there are specific terms for "the youngest child" and "the oldest child," I've seen those used far more often.
'youngest child' = /anak bungsu/. 'oldest child' or 'eldest' (if you prefer) = /anak sulung/.