Should i report my sister to the child welfare people?

she is a teenage mum, living at home with my alcoholic mother. who drinks so much she often has to be taken in to hospital. she leave the baby with my mum (tho my mum is sober when she has him so far... he is only 6 weeks old) my sister stays in bed till noon and is out parting quite a lot. she hands over the baby to any one even when he i s very sick. she all so claims Ceres allowance for looking after my disables brother , she doesn't he is left to his own devices all day(my bother is 27 and not in any danger but hes like teenager mentally) i don't think she has any responsibility in her. is this just because she is only 18? or do u think i should tell someone im really worried about splitting up the family but im extremely worried if my mother has him and she is drunk she could fall with him or hurt him in some way. is there a way i could report it with out having to give my name?would they even do anything? i live in the uk by the way pls tell me what u think i should do. im so torn.