I cant do pull ups

I've been exercising regularly for three months, and despite doing negative pull-ups and dead hangs, I still can't do a single pull-up. I can't feel the strength in my back to do one. What should I do? I weigh 83 kilos and have a 24% body fat percentage. I can do 12 reps of lat pulldowns with 45 kilos. Is my back weak, or am I just not used to the exercise? My twin weighs 70 kilos and can do 7 pull-ups. My friends started working out after me. They can now do four pull-ups from zero. Can anyone with similar experience or knowledge enlighten me and help me?

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