Assalaamu alaykum. Can you please explain how we should interpret the story of Zainab bint Muhammad, may Allah be pleased with ther, when the Prophet, sallallaahu 'alayhi wa sallam, spoke to her regarding her husband, who was also her cousin:
Then he, sallallaahu 'alayhi wa sallam, walked to where she stood and quietly said to her, “Be generous to him, he is your cousin and the father of your children, but do not let him get near you as a husband. For he is a disbeliever and that is prohibited for you.”
Kindly explain how we should behave with our cousins, are we allowed to be generous to them as the prophet, sallallaahu 'alayhi wa sallam, was said to have told his daughter to be? So what does this mean, for instance, if our cousins ever happen to meet us anywhere or at our home, then can we be polite to them and ask them to sit down or show hospitality such as offer a drink? I am confused as scholars are saying that cousins should be treated like strangers as they are not Mahrams (permanently unmarriageable kin), and I am not sure if this means that we should not even ask them how they are or be hospitable when they are in our home? How do you treat them in manners? For instance, if they are dropping something off to your home that their parents have sent as a gift? I need some clarity and advice on this please, and if the hadith of the prophet, sallallaahu 'alayhi wa sallam, about his daughter's cousin has anything to do with how we should behave with our cousins as well, in terms of generosity? May Allah reward you.

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. 

This story of Zaynab was reported by Al-Haakim in Al-Mustadrak and by At-Tabarani in his Mu’jam and by Al-Bayhaqi in his Sunan Al-Kubra, and others.

The story indicates that the Prophet commanded her to honor her cousin, and the fact that he is a non-Mahram to her does not negate the permissibility of honoring him. The fact that he prevented her from letting him approach her (as a husband) indicates observing the Islamic conditions when dealing with him because he is a non-Mahram to her.

A woman is not Islamically prohibited from receiving a non-Mahram man in her home and honoring him on condition that she observes the Islamic criteria in this regard. These criteria include the following:

Firstly, this must be with the permission of her husband if she is married. Jaabir, may Allah be pleased with him, narrated that the Prophet said in his Farewell Sermon, “Your rights over your women are that they are not to allow anyone whom you dislike to sit on your bedding (furniture).” [Muslim]

An-Nawawi said in his commentary on this statement, “The chosen opinion is that it means that your women (wives) should not allow anyone to enter your homes and sit therein (on your furniture), whether the one who is given permission to enter the house is a non-Mahram man or a woman or a man who is Mahram to the woman, as the forbiddance includes all this. This is the ruling on this issue according to the jurists.”

Secondly, she should not allow anyone to enter upon her except in the presence of one of her Mahrams or someone by whom the seclusion is negated.

Ibn Abbaas, may Allah be pleased with him, said, “I heard the Prophet say, ‘A man must not be in seclusion with a woman except if her Mahram is present with her.’” [Muslim]

As-San'aani said in Subul As-Salaam, “The hadeeth indicates that it is forbidden to be in seclusion with a non-Mahram woman; this is the consensus of the scholars. It was reported in another hadeeth: ‘For the third one (with them if they are alone together) is Satan.’ Does the presence of a non-Mahram also negate the seclusion? Apparently, the answer is yes; it negates the seclusion, because the appropriate meaning of the forbiddance is the fear that Satan may cause temptation between them.”

Thirdly, the woman must observe the hijaab if she needs to serve food to them or receive something from them and so forth. Also, she should not talk to them in a soft, attractive voice. She must avoid everything that may cause temptation. Allah says (what means): {And when you ask [his wives] for something, ask them from behind a partition. That is purer for your hearts and their hearts.} [Quran 33:53]

As-Sa‘di said when interpreting the above verse, “Because this keeps away suspicion; the farther the person is from the reasons that lead to evil, the safer he is, and the purer it is for his heart. For this reason, one of the teachings of Islam which Allah clarified in many details is that all means to evil, all its causes and introductions are forbidden, and that it is permissible to stay away from them in every possible way.”

Allah says (what means): {…then do not be soft in speech [to men], lest he in whose heart is disease should covet, but speak with appropriate speech.} [Quran 33:32]

Ibn Katheer said in regard to the interpretation of the above verse, “This means that she should address non-Mahram men in a manner in which there is no softness; i.e. a woman should not talk to a non-Mahram man in the same way she talks to her husband.”

Honoring includes all good words and actions which do not contradict the Shariah, so if she welcomes them with good words without speaking to them in a soft voice, then this is honoring, and if she cooks the best food for them and serves them the best drink, then this is honoring. Abu Hurayrah, may Allah be pleased with him, narrated that the Prophet said, “Whoever believes in Allah and the Last Day should show hospitality to his guest.” 

On this occasion, we warn against what happens in many societies of leniency regarding non-Mahram men entering upon women while not observing the Islamic etiquette in this regard, especially the relatives of the husband or the relatives of the wife, as they may be trusted but evil comes from them. The Prophet said, “Beware of entering upon women.” A man from the Ansaar said, “O Messenger of Allah! What do you say about an in-law?” He replied, “He is death itself (i.e. his seclusion with women is as serious and dangerous as death).” [Al-Bukhaari and Muslim] [In-law here means the brother of the husband or any other of his male relatives]

An-Nawawi said:

“It means that the fear of him is more than that of others, and evil and temptation are more likely to happen from him because it is easy for him to reach the woman and be in seclusion with her without anyone objecting to him doing so; unlike the non-Mahram. What is meant by in-law here means the relatives of the husband other than his fathers and his sons who are Mahrams to his wife and are permitted to stay in seclusion with her, and they are not described as death. What is meant are the brother, the nephew, the uncle, his son, and others who are non-Mahrams to her. People are generally lenient about them, and a man sits in seclusion with the wife of his brother; this is as serious as death, and it is with greater reason and more appropriate to prevent him from being in seclusion with her than a non-Mahram man because of what we have mentioned.”

Some scholars stated that the non-Mahrams meant in the hadeeth include the wife's (marriageable) male relatives as well.

Allah knows best.