Only

Einzelkind

(without sibling)

einzig

(alone in a category)

einzigartig

(singularly superior; the best)

geschwisterlos

(without sibling)

allein

(without others or anything further; exclusively)

bloos

(no more than)

erst

(as recently as)

lediglich

(without others or anything further; exclusively)

lediglich

(no more than)

nur

(without others or anything further; exclusively)

nur

(no more than)