I don't get on that well with my siblings, especially my (younger) sister - mainly because my parents' favouritism (though they always deny it and get very angry if you mention it). My sister (and younger brother to some extent) were massively indulged, whereas I as the oldest was made very responsible for things (including my parents' awful relationship) at a young age. This continues to this day, eg. my patents doing masses of childcare for my sister and during on her children whilst seeing my DC only rarely....they give (and always have done) my sister a lot of practical and financial help but are quite aggressive with me - they were very angry with me that I had a bad birth and PND for example, and kept making aggressive comments about how my sister had managed with her babies much better, had them in a routine, was up and about in Sainsburys the morning after her second baby (I had an emergency rotational forceps delivery with no pain relief and couldn't walk properly for two months but got a lot of comments about how helpless and lazy I was compared to her - thanks mum and dad :/)
I would say my parents created a bad relationship between my sister and I from very early on, because they never bothered to find out what was going on in a situation, they just blamed me as the elder child. Now I am not saying I was an angel, but I have generally a quite shy and quiet disposition whereas my sister is naturally quite like my mum - volatile and has a temper (and can be a bit spiteful). She used to get at me a lot as a child and is still given to nasty personal comments - she would push buttons and push buttons (she was a literal pincher as a child), and then have a meltdown when I eventually reacted and my parents would automatically blame me, no matter what she had said or done to contribute to an argument. I had a very intense sense of fairness and felt very angry as a child that my parents indulged her behaviour even when very extreme whilst punishing me for any arguments. I resolved that this would be one thing I never did with my DC. It creates serious long-term problems in a family if you get into a pattern of indulging one child at the expense of another.
My parents still do this completely automatically, no matter how egregious the thing is that my sister has said - for example, I was staying with them recently and my sister arrived worked up about something and as a way of letting off steam started to make personal remarks at me (more precisely, shouting at me that breastfeeding my 2 yo toddler was "disgusting" and having PND was all my own fault). I got quite upset about this, ended up crying and saying it was none of her business. My dad, who hadn't even heard the conversation, ran into the room and jabbed his finger right into my face saying "you're in the wrong, shut up, you're in the wrong". No matter what my sister says or does, it's like their inbuilt response is to blame me - as a result she has always gotten away with some of the most nasty behaviour you can think of. My brother doesn't get the same response as he's younger again and me, so it tends to be me who ends up the family scapegoat all the time.
My schoolfriends and several former partners all noticed how differently my parents treat me and my younger siblings, as it's quite obvious if you spend any time around my family.
I have been in therapy for this but it seems to have got worse again after my siblings and I had children - my parents seem to be perpetuating it by favouring my sister's children which makes me very sad for my DC.
Anyway, OP, don't do what my parents did if you want to avoid entrenching sibling divides! :)