Long summer and post partum depression

Me and my bf live with my mom, dad, 28 yr old brother, and my 6 yr old sister. I am 22. I have a 3 month old and for the summer I'm staying home with my baby and my sister. 
Little back story here: my sister is a monster. Idk what happened but she back talks my parents and just pretty much shits on everything. She's 6 and my mom was still wiping her ass... I made her start doing it by herself. Aside from that though, I understand she is 6, my mom constantly reminds me of that trust me but she talks and acts like a thirteen year old yet can't put her damn plates in the sink or pick up toys when you tell her it's time to clean up.. anyways when I was pregnant I was VERY hormonal and when she got ride or nasty I wouldn't let her talk to me like that. She would say my daughter is never allowed to touch her toys and she wasn't going to be nice to her if she did. My mom would gently talk to her but it would make me cry because we're living here and I would never let my daughter be that way to her.. just a few days ago me and my mom were telling my daughter she was so pretty and my sister said what about me and we said you both are equally beautiful and she kept insisting she was more pretty than my daughter. I didn't say anything but I just hope she doesn't grow up with that mentality because I don't want my daughter to be around someone who is full of themselves because my parents allowed her to put herself on a pedestal and talk down to everyone around her. Now that I've had my baby I have more patience with her as she's not my child and really not my problem. I'm REALLY struggling with post partum depression, maybe just anxiety and depression in general. I've let it get so out of hand sometimes I just start crying and crying and feel like I don't have a will to live if things continue how they are. I finally built up the courage and went to the doctor and was put on antidepressants and sent to therapy once a week or once a month. 
So anyways, my daughter is fussy, she's only 2 1/2 months to be exact, she doesn't really let me put her down to do much of anything. She's getting easier to distract and a little less dependent everyday, but also I find myself avoiding trying new things with her if I don't feel like I'm stable enough that day for it. My sister is just trying to help and I understand that, she's always shoving a pacifier in Marci's mouth, always waving toys in her face, always wanting to hold her or feed her. And when she is finally taking. 20-30 minute nap I just want to be alone but it turns into me taking care of my sister so I really get no time to myself. Sometimes I don't even get to take 5 minutes to eat, I have to shove food in my mouth and hold her with the other hand while Marci is still awake to even be able to eat. My parents leave at 5:30-6am and so does my boyfriend. My parents come home at 5:30pm and my boyfriend at 6 sometimes later if he has to work late. This morning at 7 my sister comes in my room while me and the baby are still sleeping and walks over to my daughters crib and starts talking to her to wake up... This wakes me up and I had just woke up with her at 6 to feed her. Now I'm wide awake and baby sleeps till 9.. I am so overwhelmed and I can't tell my sister no because I'm scared she will say I'm being mean and tell on me, and I'm scared to talk to my parents on my own because they have this shitty mentality that everything here they do for us and we don't help although we pay 300$ for a room that my bf helped my dad build as an add on, 340$ for food (my food stamps), half of cable that's in my name and we pay for the internet that we ALL use. Watch my sister whenever they want me to and she never offers to watch my baby and we go out of town over night once a month and we never leave early so we can let her sleep in but she's always asking us to come home early the next day. Find someone else to watch the baby right? Well she makes me scared to do that because she puts shit in my head like "yeah I'd be nervous to leave my baby with her" her as in my bfs mom who loves to watch her and asks to watch her even though I don't care for the woman she's a sweet lady. My boyfriend keeps telling me we need to move but then I'd have to go back to work and we would have to put Marci in daycare which I'm so scared to do... I feel so trapped. I tried to talk to my mom about it this morning and all she did was make me feel so bad I told her I didn't feel mentally stable enough for all of this and my sister was driving me crazy and she was like well I feel bad for lily because I know how you get with her... I just told you I'm too depressed but I'm watching my newborn and your disrespectful 6 year old but you feel bad for her because I put her in her place instead of talking back to me like you let her?? I sat in my rocking chair and cried as hard as I think I ever have. So hard it made my bf come home from work even though I begged him not to. Idk what I can do in this situation because dealing with this is making me worse but talking to my mom makes me feel like a failure because she talks about how she cleans dishes and washes clothes. Mind you, me and my bf buy paper plates and stuff for anyone to use because we don't have time to wash our dishes and by no means am I saying we never dirty a dish but we do try to help. And we wash our own clothes. I feel like I'm gonna go crazy. I feel so shitty about myself and either way I feel worse. I'm trying to get my life on track so I can be the mother I want to be but with all this it feels impossible. They begged us to move in so they could help and I wouldn't have to work and all this... they didn't even ask me if I was okay watching my sister this summer just assumed it would be okay. I could keep going but this is already probably too long.