Posting this on behalf of a friend, who has personal reasons to find this situation difficult.
She left her husband about 18 months ago, and has joint custody of their 6-yr old daughter. Ex-husband has got a new partner, and, at my friend's request, a carefully planned conversation was held with both my friend and her ex together to discuss his new partner with the little girl, before she then met her. Meeting this new partner went fine and the little girl is happy.
However, this last weekend the little girl suddenly met the new partner's children, most specifically her 17-yr old son. My friend feels uneasy - she would like to have been told in advance about this meeting so that she could have prepared her daughter, just as she was before she met the partner. However, more seriously, my friend is also very anxious about the boy himself - a teenager about whom she knows nothing and has herself never met, but with whom her daughter will be spending significant amounts of time?
What he would like to know is whether or not she is being neurotic, and if so, what is the way forward? As I said at the top, she has personal reasons to find this situation difficult. Is it normal to have concerns over a young child suddenly being exposed to a much older teenager, and potentially spending time alone with him, even though he has never met him and knows nothing about him?