All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
No doubt what you are facing is a trial. You have to be patient. The Prophet said: "How wonderful is the case of a believer, for there is good in every affair of his and this is not the case with anyone except the believe. If he is blessed with prosperity he thanks Allaah, thus there is a good for him in it, and if he is afflicted with an adversity, he endures it patiently, and there is a good for him in that." [Muslim]
Generally, children in their teen age are badly affected by their bad companions and by the environment in which they live. In fact, lack of a father is also a great reason that leads to unhappy incidents.
The most effective means in this regard is removing the children from such a bad environment and from their bad companions; reminding them of Allaah the evil consequences of such deeds. In addition, asking some pious teenagers to accompany your children is also a very useful means to correct their behavior.
One of the ways to change the behavior of children may be to ask each one of them separately what incites them on doing so. Being deprived of certain things may lead them to do so. In these cases, it is better to suffice their lawful needs if it is possible. You may consult a psychiatrist also to know how to deal with them.
Moreover, making supplication to Allaah to guide your children and to protect them from all sins and evils is the most important and useful solution as the Prophet said: "There are three supplications that are answered without any doubt: the supplication of the oppressed, the supplication of a traveler, and the one of a father to his children." [Abu Daawood]
As for informing your sister or her family about what happened is not compulsory, but giving back their rights is mandatory.
So, you may use any way to pay back their rights without letting them know the reality.
Allaah knows best.