Are all the children of a suckling woman considered brothers to the strange child whom their mother had breastfed, or just to the one who had suckled with him?

All the children of the woman who had suckled a strange child are considered his/her brothers and sisters.

What are the benefits of slaughtering an Aqeeqah?

All Perfect Praise be to Allah, The Lord of The Worlds, and may His Peace and Blessings be upon our Prophet Muhammad and upon all of his family and companions.

Extending thanks to Allah for His grace, expressing happiness for having a newborn, declaring lineage, and feeding the mother to compensate for the blood that she had lost during delivery.

Is it permissible for the big brother to force his sisters to wear Hijjab(Islamic wear) if their father didn`t do his part in this regard ?

The father`s attention should be drawn gently and politely, and the sisters should also be reminded with good words since Allah, The Almighty, says: “O ye who believe! Save yourselves and your families from a Fire whose fuel is Men and Stones.”{Al-Tahreem/6}.

Is it permissible for a Muslim woman to go out wearing adornment and not putting on her Hijjab(Islamic wear)?

It is impermissible for a Muslim woman to go out wearing adornment, and not having her Hijjab on.