Hi, welcome again
Oh this is common. Briefly, my older brother was diabetic from birth and my younger sister the cute only daughter. Hence my feelings I was the black sheep in the middle.
Also in relationships a woman I lived with favoured the 14yo son over the 17yo daughter. There can be odd reasons too for favouritism- my ex wifes parent lived through the war (2) and her mother lost a brother to the Germans so they named their first child of 5 the same name and of course was a replacement. Pity the other 4.
The answer? Sadly getting on your hands and knees wont help at all. That doesnt leave you many options. The option I took was to drift away and leave them all to their closeness. I joined the Air Force at 17yo and every month or so I'd return for a visit only to be reminded of the reality- less important.
The benefit long term is independence. Focus on financial security, less reliance on parents and create your own family, be it a partner, friends that treat you better than blood and people that appreciate your help.