I have a neice at 18year old,who has severe celeby palsy,confined to wheel chair,cant speak or eat by herself?

i love her so much and think of her daily,phone her 3times a week and visit her every fortnight as we stay 60miles apart.when she was 8 my sister,her mother left her wth her dad as she couldnt cope,and was a drug addict on methadone,valium,hash etc.her dad was also the same but managed to hide it from s,services for a few year,although i thought they worked closely with them.she seen drug addicts constantly in her house,life etc.when she was 10 her dad stopped me seing her thriugh being concernd about his drug add.eventually when she was 12/13 she went into restbyte,her mother never seen her a lot through the years maybe twice a year as she continued to take drugs as well as her dad,i lost contact with her from she was 12 as her dad didnt want me to see her.eventually at 15 her dad died through a heroin overdose,i managed to trace her last year arnd her 18th birthday which i am so happy about,she now has her own flat wth 24hr care,but i cant help constantly thinking of her,dreaming about her and cry often,after a year she still crys a lot when i leave her,she has no other family members visiting her although they all live near by,my mum was close to her but she died when she was 23,her other granny last seen her and spoke to her at xmas along wth her dads sister and 2 nephews,her mum is still on drugs and has seen her 3times this year,she has a computerised talker and can comuniacate to a certain level,she miss,s her dad and mum so so much as she tells me this she also asks me why her mum is like that and what happened to her dad but i just dont know what to say,i feel she is so lonely although has great carers and attends college partime,i cant help feeling guilty at what my sister is doing,i have tried talking to her but to no avail,i get very depressed about this,i am on medication myself for depression but i feel i am getting worse is there anyone out there that has had to face similiar,which i dnt think so,or can help me in anyway,do i tell her the truth about whats happening or will i always make excuses for them,its really breaking my heart,please someone give me some advice,i feel she also gets depressed through this as she loves her mum so much despite what has happened and is heartbroken about her dad as after all i mfeel he tried his best,thanks.