There's a girl in my class and im very worried?

i am a teacher at a secondary school (ages 13-17) and there is a girl in one of my classes called Natasha, she is 15. She has a 3 brothers, Liam (19) Josh (16) and Kenzie (5). Liam used to attend are school and well Josh is but is never in. Nearly two years ago the childrens mother passed away from cancer and the father who is a greek man who was very distant in the childrens life was left to look after them. Since their mother passed away their father has not been coping very well. he has been drinking excesively and on drugs. Liam then got a girl pregnant and had a baby boy and he moved out of the house because of it being so unbearable and brought Josh and Kenzie and his little boy with him. Unfortunatly the daughter Natasha remained at her fathers house, and has gone down a very bad road of drinking and drug use. Joshua their other brother who is 16 is probably in the most trouble, as he is living with the brother there are not many rules so he is able to run free and do as he likes. He has not being attending school as he has been kicked out for appauling behaviour and at the moment is on cerfew with the police and doing community service. however one of my main problems is Natasha, i was walking home with my husband at roughly 2am and i saw her passed out drunk and in lot of trouble. 1. i would like to know why she was out so late, 2. i am very concerned about why she was drinking heavily, and 3. the people she was with were people i would wish no ones daughter to be with. i am very concerned about the children and how much there life has gone down hill because i remember Liam and Josh and Natasha laughing and being very popular children, they were kind and could tell right from wrong and each was extrodinary goodlooking and beautiful. The boys were such charmers every teacher loved them and the same with Natasha. Now teachers refuse to take lessons if they know Natasha or Josh would be in it. i was thinking about going to social services about it but i have a policy as a teacher not to get involved unless asked other wise?