Why am I constantly upset ?

currently going into yr13 and I’m on summer holidays. I’m a high achieving student, and unfortunately the only reason why i am what I am is because I want to escape from this reality that I’m stuck in, in the future. I stay at home 99% of the time as I have strict parents. During the whole of the holidays, I haven’t seen my friends once (I’m not too keen on them) and I haven’t been out the house that much. Ive had family from India come around and they’re planning on living in England, so they’ve stayed at my house for a week and it’s driven me mad.
I don’t get along with my sister as we disagree on so many things and recently my mind has been so overwhelmed I can’t. And I’m just studying to take my mind off everything.
I like being alone, but yesterday, my older sister did something that hurt me and so I went away. Being alone never felt worse. So I sat with my brother and my cousin and it made me feel so much happier. I don’t know what’s wrong with me.

I'd rather you speak to a counsellor, because it sounds like you need it. As I don't know your specific situation nor do I know enough about you as a person, I am in no place to say.

It also sounds like you're an introvert, but even introverts require human connection.
I think Sigmund Freud said something along the lines of a miserable life can be down to old age (or rather aging), people, and illness. Being around the right people can significantly make your life a lot more pleasant, and it's the only one out of the 3 that you can do something about (technically you can slow down your aging and prevent illness, but it's philosophy so we're not trying to be technical).
Note: I am not specifically saying the people you're around is making you feel the way you currently do.

I know this seems stupid but what I said above kind of only occurred sometimes. I have a cousin who is my age who came from India too, and I love hanging around with her. I like talking to her and my mom. I love playing cards with them and I’m laughing all the time with them.
but it seems to be whenever my sister is added to the equation, things go down hill.
Yesterday, my sister was scrolling on my phone through my Instagram analysing everything and she tells me how stupid and ‘idiotic’ I am. I don’t know what I’m doing wrong. I’ve sat and thought about this, I try so hard but I can’t seem to do things right.

So you don't get along with your sister? Whilst it's somewhat natural to expect sibling rivalry, it's usually not healthy if it's seriously affecting you.
Again, I am not a trained or qualified mental health professional. You're better off speaking to a counsellor.

Personally, I don't see why your sister has any right to scroll through your phone and Instagram. However, that could be just me and not fully appreciating your culture.
I wouldn't know if it's something about your sister in particular (i.e. she has the problem), the way you react to your sister (a problem you can do something about), or that you 2 just don't mesh (i.e. something amongst both of you).

Her rude comments can be either of the following:

My brother and I have normal sibling rivalry, but I’d say that us two have the best relationship. We have the same humour, and like the same sort of stuff.
I get along with my mom and my dad and most people - though ofc, there are times where I might get annoyed at someone. But on the overall, I get along with people well. Honestly, I can’t build a relationship with her. As ‘weird’ as this may seem, something inside me tells me that I can’t be nice to her on the occasions when she is being genuine to me. I just can’t comprehend myself. Trust me, there’s no culture going on here with the scrolling through Instagram thing, as she doesn’t let anyone touch her phone at all. And she’s also set these boundaries between us. My brother and i sometimes cuss between each other, but my sister asks if I have any shame when I cuss… or talk about boys/ marriage as a matter of fact (though she has a boyfriend)