。
Next to her older sister, she looks short.
Literal
She [topic-は] older-sister [genitive-の] beside [at-では] short looks.
そばで ('beside, next to') marks a location, with the では combining locative で + topic は to set up '*as for that location*, ~.' The pattern frames a contrast: in this place vs. presumably elsewhere. 低い here means 'short (in stature)'; the adverbial 低く modifies 見える ('appear, look, seem') for the pattern ~く見える ('look X / appear X'). Note 低い is the same word for 'low' (low ceiling, low pitch) — context picks the body-stature reading.