parse it like this:
「いつになったら結婚するのか」も 聞いといてね
Oh, and also ask (her) when is she planning to get married.
Edit: for のか used as ending of question sentences, check this answer
P.S. 聞いといて is colloquial for 聞いておいて. I think we have other questions on the 〜ておく form but I don’t have a link handy.