If his non-Muslim relative says to him I love you, can he respond in a like manner?
Question

I was wondering what a Muslim should say to a non Muslim family member who says, "I love you" to him or her. Should we say I love you too in return? Are we allowed to say that to our non Muslim relatives?
Praise be to Allah.Firstly:
Submission to the Lord of the Worlds, body and soul, is one of the greatest objectives of the message of Islam.
One of the implications of that is that the motive for loving whomever you love and hating whomever you hate is seeking the love and pleasure of Allah. Thus a person makes the religion of Allah the focus of all his feelings and all his actions.
The Prophet (blessings and peace of Allah be upon him) said: “The strongest bond of faith is that you love for the sake of Allah and hate for the sake of Allah.”
Narrated by Ahmad (18524); classed as hasan by the commentator on al-Musnad. Also classed as hasan by al-Albani in Saheeh at-Targheeb (3030).
It was narrated from Anas ibn Malik (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “There are three things, whoever attains them will find the taste of faith: when Allah and His Messenger are dearer to him than all else; when he loves a man and does not love him except for the sake of Allah; and when he would hate to return to disbelief as much as he would hate to be thrown into the fire.”
Narrated by al-Bukhari (16) and Muslim (43).
Secondly:
What is affirmed above does not mean that there cannot be, between a Muslim and a particular disbeliever, a natural kind of love for some reason, such as ties of kinship, ties through marriage (in-laws), or acts of kindness and favours on the disbeliever’s part, and the like, whilst still disavowing his religion and being opposed to him on that score.
Allah, may He be exalted, acknowledged the love of the Prophet (blessings and peace of Allah be upon him) for his uncle Abu Talib, despite his being a disbeliever. Allah, may He be exalted, said (interpretation of the meaning):
“Indeed, [O Muhammad], you do not guide whom you like, but Allah guides whom He wills. And He is most knowing of the [rightly] guided.” [al-Qasas 28:56].
That love for his uncle was a natural kind of love, because he was a relative.
Allah has permitted marriage to women of the People of the Book, even though marriage generates love between the spouses, as Allah, may He be exalted, says (interpretation of the meaning):
“And of His signs is that He created for you from yourselves mates that you may find tranquillity in them; and He placed between you affection and mercy. Indeed in that are signs for a people who give thought.” [ar-Room 30:21].
This love is a instinctive kind of love, like the love of food, drink, clothing and so on. That does not mean that religious opposition cannot coexist with a natural love, because the two issues are different.
This is like medicine; a person can love and hate medicine at the same time. It is loved in one sense and hated in another sense.
Similarly, fighting for the sake of Allah involves natural dislike of what it involves of pain, but Islamically-prescribed love for what it brings of great reward. Allah, may He be exalted, says (interpretation of the meaning):
“Fighting has been enjoined upon you while it is hateful to you.” [al-Baqarah 2:216].
Ash-Shaykh Ibn ‘Uthaymeen said:
There is no blame on a person if he dislikes what has been ordained for him, so long as he does not dislike it because the Lawgiver has enjoined it, rather he dislikes it because that is a natural feeling. But with regard to it having been enjoined by the Lawgiver, what is required is to accept it and be content with it.
End quote from Tafseer al-Fatihah wa’l-Baqarah (3/50).
Based on that, a son or a cousin may be a disbeliever, but he respects the rights that are based on kinship and treats his Muslim relative or father kindly, and upholds the ties of kinship with him perfectly.
A wife may be from the People of the Book, but she is loving towards her husband and treats him kindly, so undoubtedly he will love her. This is a natural kind of love, but at the same time he should hate what she follows of false religion. This is the meaning of opposing her with regard to her religion, or disavowing her religion.
Ash-Shaykh Salih Al ash-Shaykh said:
The point is that al-wala wa al-bara (loyalty and disavowal) with regard to a specific disbeliever is of three levels:
1.       Love and loyalty towards the disbeliever because of his disbelief. This in itself constitutes disbelief.
2.       Loving him and honouring him for purely worldly purposes. This is not permissible and is prohibited, and it is a type of blameworthy loyalty.
3.       When it is in response for a favour, or it is because of ties of kinship. This kind of love or doing acts of kindness towards him because of that and the like, so long as he is not one of the disbelievers who are in a state of war with the Muslims, is the kind of love concerning which there is a concession.
End quote from It-haf as-Sail bima fee at-Tahhawiyyah min Masail, p. 501.
Based on that, there is a difference between loving a specific disbeliever because of his religion and what he is following of falsehood, and loving him for a particular reason, such as ties of kinship or marriage. There is nothing wrong with the latter, as it is not contrary to opposing them because of religion and disavowing their disbelief.
If the basis for this love is permissible according to Islamic teaching, and is not forbidden according to Islam, then there is nothing wrong with expressing it, especially if that is by way of responding in kind.
And Allah knows best.