"Is that your wife?"
When you ask a shi bu shi, x bu x, that is the questions, so ma is not needed.
那是你的妻子吗？would be a better translation; Is that your wife?
You use "是不是" to ask a question, when you already assume an answer.
So for example: 那是不是你的妻子？; Isn't that your wife? 你是不是要茶? ; Don't you want tea? ect..
Why this app doesn't provide the pinyin system? It would be useful for vocabulary and pronunciation...
Wrong. "Isn't that your wife" would 那不是你的妻子吗? 是不是 is just a different way to ask a regular question which just deosn't require a 吗.