。
She'd better sit next to me.
Literal
She [topic-は] my side [at-に] sit [it's-better-ほうがよい].
~ほうがよい (or ~ほうがいい) is the standard pattern for giving advice or recommendations — 'it's better to do X.' Attached to the dictionary form of a verb (here 座る), it expresses the recommendation that the action be taken. Slightly more formal than ~たほうがいい, which uses the past form and tends to be the more common conversational choice. 横 here means 'side / next to' — overlapping in meaning with 隣 but slightly more general (隣 typically implies immediate adjacency).