assalamualikum to all ,
i hope and wish all are in the best state of imaan , allah swt does tests our imaan in all the conditions , my sis was expecting and we were so happy alhamdulillah but allah swt had some other plans and the baby coudnt make it (Inna lillahi wa inna ilayhi raji'un) , may allah swt give her and all of us strength and may allah swt bless her ameen .
Walaikum Ssalaam warahmathullaahi wabarakaatuh,
Innalillaahi wa inna ilaihi raajioon.
Hope your Sister is in good health and recovering from it. Its painful. May Allah give your sister and brother in law, the courage to accept it, support each other in such tough times and hope for the best. Its not easy to be a parent and undergo this, I think. It leaves a life long scar in most. May Almighty bless the couple with children sooner.
Even prophet

cried , when he lost a child/grandchild.
It was narrated that Usaamah ibn Zayd said:
We were with the Prophet (blessings and peace of Allah be upon him) and one of his daughters sent word to him, calling him and informing him that a child of hers – or a son of hers – was dying. He said to the messenger: “Go back to her and tell her that to Allaah belongs that which He has taken, and to Him belongs what He gives, and everything has an appointed time with Him. Tell her to be patient and seek reward.” Then the messenger came back and said: “She is adjuring you to come to her.” The Prophet (blessings and peace of Allah be upon him) got up, and Sa‘d ibn ‘Ubaadah and Mu‘adh ibn Jabal got up with him, and I went with them. The child was lifted up to him and his soul was rattling like water poured into a waterskin. His eyes filled with tears and Sa‘d said to him: What is this, O Messenger of Allaah? He said: “This is compassion that Allaah has instilled in the hearts of His slaves. Allaah only shows mercy to the merciful ones among His slaves.”
[Al-Bukhaari, 1284; Muslim, 923]