Nepotism is obviously haram when it leads to other haram things such as injustice or giving guardianship over public interests to those who are incompetent or corrupt hence causing harm. There can be several relevant texts such as the following:
يا أيها الذين آمنوا كونوا قوامين بالقسط شهداء لله ولو على أنفسكم أو الوالدين والأقربين إن يكن غنيا أو فقيرا فالله أولى بهما فلا تتبعوا الهوى أن تعدلوا وإن تلووا أو تعرضوا فإن الله كان بما تعملون خبيرا
O you who have believed, be persistently standing firm in justice, witnesses for Allah, even if it be against yourselves or parents and relatives. Whether one is rich or poor, Allah is more worthy of both. So follow not [personal] inclination, lest you not be just. And if you distort [your testimony] or refuse [to give it], then indeed Allah is ever, with what you do, Acquainted.
— Quran 4:135
إن بني إسرائيل كان إذا سرق فيهم الشريف تركوه، وإذا سرق الضعيف قطعوه، لو كانت فاطمة لقطعت يدها
If a reputable man amongst the children of Bani Israel committed a theft, they used to forgive him, but if a poor man committed a theft, they would cut his hand. But I would cut even the hand of Fatima (i.e. the daughter of the Prophet ﷺ) if she committed a theft.
— Bukhari
And it is also evident from more general texts such as:
إن الله يأمركم أن تؤدوا الأمانات إلى أهلها وإذا حكمتم بين الناس أن تحكموا بالعدل
Indeed, Allah commands you to render trusts to whom they are due and when you judge between people to judge with justice.
— Quran 4:58
ما من عبد استرعاه الله رعية، فلم يحطها بنصيحة، إلا لم يجد رائحة الجنة
Any man whom Allah has given the authority of ruling some people and he does not look after them in an honest manner, will never feel even the smell of Paradise.
— Bukhari
إذا أسند الأمر إلى غير أهله، فانتظر الساعة
When authority is given to those who do not deserve it, then wait for the Hour.
— Bukhari
Because Nepotism involves giving positions without regard to ability.
As for the athar of Umar which you have quoted, it is disconnected as Abdul Malik ibn Umair did not meet Umar. Also keep in mind that Umar is talking about public offices such as governors and tax collectors and not about private employment. The ahadith in Mu'jam al-Kabir Tabarani or Mustadrak al-Hakim are also related although they have weakness.
