。
Why don't you bring your sister along?
Literal
Younger-sister [object-を] take-along come-[honorific-おいでになって] [suggestion-はどうですか].
おいでになる is the honorific form of いらっしゃる/来る/行く — very polite. ~てはどうですか ('how about doing X?') is a polite suggestion. 妹さん uses さん as an honorific suffix — the speaker is referring to the listener's sister respectfully.