All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Islam urges Muslims to uphold the bonds of brotherhood between them and made it clear that this is among the requirements of faith. Allaah The Almighty Says (what means): {The believers are but brothers.}[Quran 49:10] This is more confirmed to relatives through a marital relationship, due to that relation between them. So, you should all seek to settle your differences. Allaah The Almighty Says (what means): {…and settlement is best.}[Quran 4:128]
On the other hand, it is not binding upon you, under Sharee‘ah, to invite the parents of your daughter-in-law to that wedding, and had you done, perhaps it would have been better. However, she should not have become angry because of your abstention from inviting her family. Also, she is not required, under Sharee‘ah, to remain with you in the same house, rather she has executed what she was ordered to do by her husband, whom she is required to obey in what is good. But it is impermissible for her to deprive you of maintaining kinship ties with those two children, for by so doing, she helps sever kinship ties.
In fact, we do not know what you mean by saying that they had separated your son from you. But in any case, it is due on your son to maintain kinship ties with you. The scholars stated that it is due on the son to spend on his parents in case they are poor and have no earnings, since this is among the aspects of dutifulness and kindness commanded by Sharee‘ah. For further information, see Fataawa 82254 and 87019.
However, we alert to what we have previously mentioned, i.e. to seek conciliation and be tolerant, and seek the mediation of righteous persons when it is needed. Allaah The Almighty Says (what means): {No good is there in much of their private conversation, except for those who enjoin charity or that which is right or conciliation between people. And whoever does that seeking means to the approval of Allaah - then We are going to give him a great reward.}[Quran 4:114] It is narrated on the authority of Abu Ad-Dardaa’ that he said: "The Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: "Should I not tell you about what is better than the degrees of fasting, prayer and charity?" They said: "Yes." He, sallallaahu ‘alayhi wa sallam, said: "It is to reconcile between people, for spoiling kinship ties between them is the shaver." [Ahmad and At-Tirmithi] “The shaver” means that it destroys one’s religion as shown in the wording of At-Tirmithi.
Finally, it should be mentioned that it is binding upon the Muslim to seek judgment from rulings of Sharee‘ah and it is impermissible for him to seek judgment from man-made laws.
Allaah Knows best.