My sister the drug addict

Hi,im going to call myself Vikki.Im new to this site and I'm really looking for advice. I have a sister who has been a drug addict since the age of 12 and she is now 33.She now has a son,my nephew who is 5.They both stay with my mum as my sister wasn't coping by herself.My nephew was born addicted to heroin andwhen hefoot home from hospital he went to live with my sister and her boyfriend (his dad.) This was a volatile relationship as both were drug users.My mum was notified about a fight at their house when my nephew was 3 and my mum removed him from the property.He was then placed on the at risk register and my mum was allowed by social services to keep him but my sis had to show up atmums totake him to nursery every day and out him to bed at night.This was fine at first but then my nephew was breaking his heart all the time for his mum so my mum decided to let my sister move in with her and my nephew.This was ok at first too but over past couple of years my sister has left all responsibility to my mother.She never takes him anywhere,does anything with him .Never buys him anything but in her head she genuinely thinks she's a great mum.My mum is in her mid 60sworksfull time in a pressurised job.Shes a wreck living with my sister as he shouts.screams ans basically thinks that she as well as my nephew is my mothers responsibility.She gives my mum nothing financially but still expects everything from clother to toiletries to be bought for her.She has put my mum through living hell for 20 odd years and I despise her with all my heart.As long as my sister has breath in her ravaged body my mum and nephew have no chance of living a normal decent life.I hate her for everything she's put my family through and continues to do so. I want to know if anyone knows what position my mother stands in legally in regards to having my nephew.My mums scared to take my nephew and livewithin me,my husband and my 6yrold son as she says the police w I'll come and remove my nephew from us and give back to my sis.Wgat can we do legally to get guardianship permanantly of my nephew.She is not fit to look after him.Why the hell do social services seem to insist on keepinfakirs with drug addict parents?????They are not fit and these kids almost always grow up with issues of their own due to their parentsparenting or should I say non parenting ways.My nephew is only 5 and already has issues.He is so timid,nervy and anxious as he can't do right for doing wrong in my sisters eyes.oPoor wee soul has zero confidence.He loves his mum to death and he always will but surely to god he deserves a chance in life.He starts school in August and I dread to think of him making new friends and asking them to come for sleepoverfoams what parent in their right mind would let their kid go stay in a drug addicts house.People go on about addicts having rights but do you know what ,what about all these poor wee kids being raised in homes with addicts as parents.Wheres their rights?????Sorry for the rant but I just need help in finding out what we can do to get  my nephew living witmy his granny.