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 ?

All Perfect Praise is due to Allah, The Lord of The Worlds

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 (What means): "O ye who believe! Save yourselves and your families from a Fire whose fuel is Men and Stones." [Al-Tahreem/6]. And Allah Knows Best.

Is abortion permissible when the family isn`t ready to receive a new baby?

Abortion is forbidden unless it constituted a serious threat against the mother`s life based on the opinion of trustful and God fearing physicians.

Is Zakah(obligatory charity)due on an indebted merchant?

Debt doesn`t abrogate the dueness of Zakah, thus a debtor who has money, articles of merchandise, or other Zakah funds at his disposal should pay Zakah from them.

Is Aqeeqah due on the miscarried embryo?

No Aqeeqah is due on the dead embryo.