If one is offered to date a girl and one knows that a friend of his dated her sister in the past and had some issues with the parents, can he ask the person about it, for constructive reasons if one is worried? Or is this lashon hara?
It is permitted to ask about this matter.
The Chafetz Chaim lists a number of conditions for permitting speaking negatively about people, the first of them being that the intention should be for a constructive purpose (the others are that there is no other way to find out, that the facts are known first hand, that no exaggeration is made, and so on).
There is no more constructive purpose than making a shidduch, and it is permitted, for this purpose, to inquire as to the details of probelms you have heard about.