بسم الله الرحمن الرحيم
(Fatwa: 942/762/D=11/1437)
(1) Your sister and brother should remember the following dua and read out it extensively specially after salah:
رَبِّ هَبْ لِي مِن لَّدُنْكَ ذُرِّيَّةً طَيِّبَةً إِنَّكَ سَمِيعُ الدُّعَاء [آل عمران : 38]
In-shaAllah with the blessings of this verse Allah shall bless pious child.
(2) Also recite the following dua after each salah, in-shaAllah you shall have child:
رَبِّ لَا تَذَرْنِي فَرْداً وَأَنتَ خَيْرُ الْوَارِثِينَ [الأنبياء : 89]
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband