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 a woman`s face an Awrah (a private part) that should be covered?

During prayer, a woman`s whole body is a private part except the hands and the face. Outside the time of prayer, scholars are of the view that a woman should wear on her Islamic attire to avoid temptation.

I`m getting old and haven`t got married yet. I love wearing the veil, but people advised me to take it off because it could stand in the way of my marriage, and that I should wear it after getting married, what should I do?

The veil (face drape) is the wear of the Prophet`s (PBUH) wives and righteous female companions (May Allah be pleased with them) and whoever imitates a certain group of people shall receive the same reward or punishment like themselves. Only those who are religiously poor deny women wearing the veil, and thinking that it stops a woman from getting married is from the evil suggestions of the devil.

What is the ruling on someone who doesn`t perform prayer?

All perfect praise be to Allah,The Lord of The Worlds                                                                                                                                                                    Not performing prayer is a major sin, and one who doesn`t pray out of laziness is considered an oft-sinner ;whereas, the one who denies that it is mandatory is considered a disbeliever. And Allah Knows Best.