AIO Constantly expected (26 F) to play with little sister (7)

I have a half sister and she's my dads and step mothers child. She's very cute and I do love her but im starting to resent her.

I dont go round much anymore but when I do both parents disappear and leave me with her - which would be fine but they don't warn me and I then can't find them. Ive looked after her from a baby so I know the ins n outs and how to care for her / play with her.

The problem is - I rarely see my dad anymore. I barely saw him before due to divorce and him being obsessed with his work. But now what little time he gave me is now taken up by my little sister.

When I am with her she begs me to play with her and my dad just sort of goes "well we'll be dead so she needs to have a good bond with you and your sister when shes older" but my older sister lives in a different country and never comes back they have to go to her and i know she barely spends time with little sister. My dad makes more time to hang out with older sister alone because they have a startt up company together.

I am autistic and easily overwhelmed by repetitive language, irritating sounds or painful sounds, unexpected touch when im stressed or in a heightened state - which is why I have decided not to have children since its unfair on that child to have a mother who can't meet their needs. I know if this is my life 24/7 I would snap in some form. So far i havent with little sister but she knows. Ive gotten mildly pissy with her or shot her glances when shes being a brat n screetching (noise) or quite often to get your attention she'll repeat over and over again. I love being her sister and I love playing with her/ looking after her - on my terms. Im not a good sister when im exhausted and overstimulated which i am all the time because of how im treated in dad's house.

My step mother (who's idea it was and she was desperate for a child apparently- tho it was never even hinted at my older sister or I that she wanted a child) and my dads super chill so went along with it. She cant handle motherhood and seems to have hated every step - she was 48 when she got pregnant. She doesn't like it takes up all her time so my dad does most of the drop offs, taking to activities and such. Which means our hang out time is always with my little sister, always doing sometbing she needs to do or giving her attention in some form of way which means I cant have adult discussions with my dad. When they had her I was at a time in my life I needed a parent and sadly my own mother wasnt really there for me though we have now gotten better and have a better relationship. Though she does bitch about my dad and my older sister all the time still and uses me as a therapist to talk about how my older sister moved countries just to spite her etc... (my older sister is her fave - I say this because everything's always "she wouldnt like that" as an example). In fact she brings her up so much her partner (my step dad) who rarely parents me through respect of my own real father spoke up and said something to her.

So I didnt have either parent and throughout my entire 20s thus far I feel so alienated from my entire family. Im also the black sheep. I did drugs, went to raves. No one was really checking up on me so when id moved out at 18 and no one heard from me in 6/7months my mother was the only one to check on me. I quit doing drugs daily but relapsed again recently but have now stopped once more. I'm now going on a backpacking trip I dont plan on coming back from since im moving to a different country and I need a fresh start away from all this. My family also dont really support my dreams and laugh about them so I just need to get away.

I love them all so dearly but I feel like im in a fish bowl watching but never truly interacting below surface level. Ive spoken to my dad about this but he gives me the same answer every time "im gonna be dead and you'll be her only family". He's a very blunt man and says what he thinks but hes also very light hearted and jokey. The thing is I really will be her only family because my older sister is obsessed with my dad and constantly seeks his approval/attention. So she absolutely resents my little sister. So I really am gonna be her only family and that terrifies me. I never wanted all this responsibility- I've said for the longest time I never want kids. The way my autism is i am social and extroverted but I need my own time and a lot of it by myself. I dont even date anymore because I find fulfillment from my friends and having a partner means sharing a space.

Am I a horrible person for saying these things? I feel like i should just suck it up n help out since theyre both aging parents and its scary. My dad collapsed when she was about 2 or 3 from lack of sleep. Hes aged so much so quickly. I feel like my step mum made this decision and I know my dad tried to warn her over n over but she just wouldnt listen even tho he went through it twice. The worst thing is even tho he complains to me when its just us two he makes statements like "rearing a child is much easier when you actually like their mother / get on with their mother" etc. My entire blood related family (my dad less so though) talk about eachother behind their backs all the time. My older sister who's 28 blames our mum for all our shortcomings - yes she was emotionally abusive growing uo and we had a hell of a childhood but our mum has taken a lot of steps to become a better person and sure she has dips but shes trying.

I also just dont see the point in bitching about one another. It creates such an anxious n toxic family environment where somehow im always the middle man.

Things my parents say to me: "How's older sister? She's not doing too good so you need to fly to her country to check on her" "Older sister hasn't spoken to me can you find out how she is" My older sister has even come to me after a massive falling out with bith parents recently n asked me to help. But everyone lies so ive no idea who to believe. And then im sat on the floor, empty inside feeling like im watching the world through a screen as my little sister plays n tells me im not playing right or I need to do this or that. Sorry needed to get this rant out.

These are my questions.

Should I just be helping my family since they're family? Am I overreacting?

Finally, if i am being rational - do I just go on this backpacking trip, get settled in this new country whilst keeping my communications with all of them very very minimal?

I feel so alone atm but im getting to a point id rather have no one around me than all this because its eating away at me.