I need advice. i am 31 and have a 23 year old ...

i need advice. i am 31 and have a 23 year old sister with a 2 year old. i myself am married and have a 3 year old. my sister has cause my parents and the rest of our family grief since she was around 14 years old. she's run away a handful of times, lies incessantly, shoplifted, has tendencies to call the police irrationally on whomever and whenever she feels like it, etc. she is currently split up from her husband as of around 9 months ago. she began going out and staying out late probably well over a year ago. she met someone and cheated on her husband and left him. believe me he is not innocent in this either. anyway she has been diagnosed with bi-polar and i am pretty sure she inappropriately uses pain killers. she left her baby girl with my mom about 8 mos ago and barely waltzs in and out of the childs life. the baby's father is just as pathetic and doesn't see the child and or support the baby in any way. about 3 mos ago my parents who may be equally dysfunctional began fighting a lot a going through hard times so in turn my niece has been left in my care. my niece is experiencing some major behavioral problems which make home life for me and my son difficult. i love my niece more than anything and have been trying to get my mom or my nieces mom oreven my 21 year old sister to help me out a few hours during the day. my 3 year old is going through yet another cycle of seperation anxiety and is always crying for me and i feel like i'm torn because both of theses babies need me. it is very difficult and has left me worn out with anxiety and gaining weight and feeling irritable and sick all the time. i'm a really happy person and i'm really a good mom to my son who is my world but i keep finding myself using a tone with him and being irritable and crying all the time. i feel like i'm a prisoner in my own home because i can't take my son out anymore. he is used to going to the zoo, childrens museums, parks etc. but i can't safely watch these two out on my own. my niece hands down does not listen and they would both take off in two seperate directions. i feel i am the baby's only hope. i love her and care for her like she is my own, i just don't know if i can keep on doing it at the risk of my familys quality of life. i feel so selfish and disgusting. i don't know what to do. every night before i go to bed a wave of dread comes over me of what the next day will bring. please any advice would help me!! thank you