Sikhism is not a religion. Shri Guru Gobind Singh started a Mission and named its people as Sikhs. Its sad, people forget the foundation and purpose of this holy mission and start treating it as a religion.
It is a diatomic religion, but it also recognizes Abrahamic prophets though Sikhs do not give any importance to Koran, Torah, bible, Vedas, Ramayana, Sutras etc but they can read them to broaden their understanding. Sikh prophets brought the revealed word to us. It is a mission, that is true which has its own code of conduct, prophets, holy book, temples, schools, preaching class, therefore it is also a recognized religion. You can find similarities with both:
Like Abrahamic religions Sikhs believe in one God
Like Muslims we do not believe in idol worship and caste ism
Like Dharmic religions Sikhs believe in reincarnation, Sikh concept of reincarnation is different than others though.
Like dharmic religions we do have cremation, cremation here is a preferred method though not necessity.
Sikhs do not believe in fasting, pilgimmage, ghost worship, satans etc.
Sikhs do not have any Kaffir or malechhas,
Sikhs do not believe they are chosen people, they believe come what way, God judges you by your deeds not by religion.
Like dharmic religions, we believe in having Guru, but Guruship in Sikhism is conferred to holi book, granth, so it is called Guru Granth Sahib.
Sikhs believe in charity work and fighting to defend someone's rights. Guru Arjun Sahib and Guru Teg Bahadur sacrificed their lives for rebel muslims and Hindus respectfully.
Its Dharmic because Sikhs are born out of Hinduism - To understand Sikhism you have to understand Hinduism. Hinduism is not Christianity and Islam which are codified religions, and have a book and a prophet. Hinduism has originated from various rituals tribal customs and social life over the centuries. Hinduism has 1000?s of Gods and Goddesses and various diverse thoughts.
Predominant amongst these thoughts is a way of life called Sanatan Dharma which more than 60% of the Hindus follow, and this is generally mistaken as Hinduism, and the other thoughts are treated as offshoots of Hinduism. The real truth is that Hinduism means Sanatan Dharma AND all the other thoughts. Sikhism is but one of these thoughts.
Firstly, the British with their policy of divide and rule, and secondly, political turmoil and hatred between Hindus and Sikhs has put the idea of a separate identity in the mind of us Sikhs. Many of my co-coreligionists will say ? but we have different ideas as compared to Hinduism. Sure - I agree that we are different in ways but there are 50 ? 100 major different thoughts within Hinduism and we are BUT one of them. This paragraph answers your questions - all the Dharmic thoughts are different from each other in one or two unique ways.
Do you understand ? Whereas fundamentalist Sikhs we say that we are 1 amongst 7 great world religions the fact is that there are actually 4 great religions. One - Christianity has various branches - Catholic, Protestant, Methodist, Adventist, etc. Second ? Islam has various branches - Shia, Sunni, Ahmediya etc. There is Judaism and finally Hinduism ( which is a collection of various beautiful thoughts like Buddhism, Sanatana Dharma, Sikhism, Jainism, Arya Samajists, Vaishnav, Shaivaites Manbhum, Santhal etc )
So Sikhism is born out of Hinduism. Guru Nanak was born a Hindu, he was a traveling holy man who followed a custom of Sant Mat ( or Saint Tradition ) quite popular amongst saints those days. It was a thought he inculcated in his believers and told them to learn it. His believers were called Sikhs as Sikh means ? ?to learn? We had 9 more Gurus. Yes Guru Nanak also took some minor feedback from Islam but that is the nature of Hinduism ? to absorb from everyone.