All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Prayer is an obligation only on a person who is competent for religious assignments by being adult and sane. Therefore, if your sister has not yet reached puberty, she is not obligated to perform the prayer but she should be taught (how to pray), and if she is pubescent, she is obligated to perform the prayer. For more benefit on the signs of puberty, please refer to Fataawa 87018 and 83431.
Since she is of sound mind, she is considered competent for religious assignments and she is obligated to perform the prayer and ablution according to her ability, because a person is not exempted from the prayer as long as he/she is conscious as we clarified in Fatwa 84293.
However, if she is unable to stand when performing the obligatory prayer, then she should pray in her bed as the Prophet said: “Perform the prayer while standing and if you are unable, then pray sitting, and if again you are unable, then pray (while you are) lying down on one of your sides.” [Al-Bukhari]
The fact that she involuntarily urinates does not exempt her from the prayer and the ruling of a person who has incontinence of urine applies to her. If this always happens to her, and it is difficult for her to avoid it because it is frequent or continuous, then she should wash the place very well and put a pad or the like around it, and then she should perform ablution after the time of the prayer starts. Then she should pray the obligatory prayer and as much as supererogatory prayers as she wishes, and if she has any urine discharges, then this does not harm her even if it is during the prayer. When the time of the following prayer comes, she should perform ablution again and wash her clothes that might be stained by urine and she should do like she did the first time.
However, if she has incontinence of urine at intervals and then urine stops, and it is possible to wash her private parts and perform ablution and the prayer during this time of the urine flow interruption, then she is obligated to delay the prayer until the time of the urine flow interruption as long as she does not fear that the time of the prayer will expire. However, if she fears that the time of the prayer will expire, then she should pray after performing ablution as we previously clarified. For more benefit, please refer to Fataawa 85661 and 89504.
Allaah Knows best.