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.
First of all, you did not mention to us the reason which prevents you from informing your sister about the matter and that this child is handicapped.
Therefore, there are two probabilities:
1- The reason might be the shock and your sister might be affected by this child being handicapped. If this is the case, then in our view she should be gradually informed about it in a gentle and nice manner, and she should be reminded that Allaah may bless her with much and substantial good thanks to having that child who is handicapped. Allaah Says (what means): {But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allaah Knows, while you know not.}[Quran 2:216] For more benefit on the excellence of patience, please refer to Fataawa 83577 and 84468 on the reward of being patient on being afflicted with a handicapped child.
2- The reason might be your sister’s sickness and the like, the thing which may shock her in a way that she will be harmed by such news. If this is the case, then it is better to wait until her situation gets better and in order to repel the expected harm. Indeed, it is confirmed that the Prophet said: “There should be no harm and no reciprocal harm.” [Ahmad and Abu Daawood]
Allaah Knows best.