My brother owns a takeaway and my other brother works there sometimes. Sometimes I have to do deliveries and I get tips but my brother doesn't pay me nor do I get a salary. The only money I earn is tips from this. I do not ask for tips or anything, the customers just give me it . Is this money halal? Also I am not an employee. This is a family business so I help out but I don't want money as I just want to help but sometimes customers just give tips and I insist not to but they gave it out of kindness as some houses are tricky to find. Not taking the money would be rude if I did that.
Some scholars consider tips as haram, saying:
The difference between a haraam gift and a permissible gift is that if it is given because of a person’s work, then it is haraam. The guideline concerning this is to look at the person’s situation: if he was not in this job, would he have been given this gift? This is what the Prophet (peace and blessings of Allaah be upon him) meant when he said: “Why doesn’t he sit in the house of his father and mother and see if he will be given anything or not?”
So if these tips somehow reach the level of a bribe then they are haram.
Also, tips have negative effects which should be taken into account (for details read islamqa #82497).
Now on the other hand receiving tips is allowed if your employer is aware of it and accepts it. In your case your brother doesn't even pay you as he seems to be aware that his customers would give you tips here I'd say would apply the statement of this hadith compiled in sunan abi Dawod, sunanan-Nasa'i and al-adab al-mufrad of al-Bukhari:
...and requite him who does a favour to you, but if you are unable to requite him ...
(For details for the exception of tips if the employer is aware of read islamqa #143737)