If I hear a non Muslim swear, or backbite or do any other sin do I have to telll them not to do it? And am I assisting in sin if I help carry haram foods for my non Muslim mother to eat?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If the food that you will help to carry to your mother is lawful, then there is nothing wrong with you in helping with that. This is a kind of kindness and dutifulness for her.

A Muslim is ordained to be kind and dutiful to his parents, even if they are non-Muslims. Allah Says (what means): {But if they endeavor to make you associate with Me that of which you have no knowledge, do not obey them but accompany them in [this] world with appropriate kindness.} [Quran 31:15]

Besides, Asmaa’ bint Abi Bakr reported, “My mother came to me during the lifetime of the Messenger of Allah and she was an idol-worshiper. So I asked the Messenger of Allah ‘My mother has come to me, and she desires to receive a gift from me; shall I keep good relations with her?’ He said, ‘Yes, keep good relations with your mother.’” [Al-Bukhaari and Muslim]

As regards what is forbidden, such as wine (all kind of intoxicants), or pork, and so on, then it is not permissible for you to give it to her or to help her in this because the disbelievers are addressed by the branches of the religion and it is not permissible for us to help them in committing what is forbidden.

If you hear a Kaafir backbiting or committing a sin, then you do not have to forbid him from doing so.

Ibn Rajab said in Jaami’ al-‘Uloom wal-Hikam: “Imam Ahmad said: “A Muslim is not obliged to advise a Thimmi, he is only obliged to advise a Muslim, as the Prophet said: "…and advice is for every Muslim.” [End of quote]

What is required to do is to leave this meeting where Allah is disobeyed as Allah Says (what means): {And it has already come down to you in the Book that when you hear the verses of Allah [recited], they are denied [by them] and ridiculed; so do not sit with them until they enter into another conversation. Indeed, you would then be like them. Indeed Allah will gather the hypocrites and disbelievers in Hell all together -.} [Quran 4:140]

Al-Qurtubi said in his Tafseer: “This indicates that it is obligatory to avoid sinners when they commit evil because the one who does not stay away from sinners is in fact pleased with what they are doing. Being pleased with disbelief is disbelief. Allah says {Indeed, you would then be like them.} All those who sit in places where sins are committed, and don’t attempt to change the sin by forbidding the evil, are equal in sin. They should forbid them if they speak about a sin and did it. If they can't do this, they should leave these places at once so as not to be among those who were mentioned in the previous verse.”

For more benefit, please refer to Fatwa 343228.

Allah knows best.