Translation:She is not less pretty than her older sister.
"Tan bonita como". En español no estás diciendo que es igual de bonita que su hermana, si no que es menos bonita(less)
'She is not uglier/less prettier than her sister' would be better English, but maybe future updates of DL might accommodate this...
No. That is the opposite of the intended meaning. "Ella no es menos bonita" means "She is NOT less pretty" or "She is prettier". As Spanish doesn't have "-er" and "-est" adjectives (e.g. prettier, prettiest), they use other means to get their point across.
Therefore, the correct translation is "She is not less pretty than her older sister" or "She is prettier than her older sister". Hope this helped you.