When does the time of Isha' prayer end?

All perfect praise be to Allah,The Lord of The Worlds                                                                                                                                                                        It ends at the emergence of true Fajr (Fajr Al-Sadiq ), and when the second Athaan of Fajr starts.And Allah Knows Best.

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 should I do in order to wake up for Fajr (dawn) prayer?

All perfect praise be to Allah, The Lord of The Worlds                                                                                                                                                                What should I do in order to wake up for Fajr (dawn) prayer? Answer: Some of the acts that aid in offering Fajr prayer are: 1- Going to bed early. 2-  Willingness to wake up. 3- Reciting Al-Korsi verse from Chapter Al-Baqarah. 4- Taking refuge in Allah from the cursed devil, and making ablution immediatley after being awake, but it is preferable to ask someone to help in this regard to perform the Fajr prayer. And Allah Knows Best.

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.