Does one become deprived of the angels' presence and their attendance at his prayers and recitation of the Qur'an due to the presence of pictures and cartoon drawings on his sister's clothes in the same room, and is he sinful for praying in this room despite the availability of other rooms?

It is forbidden to wear anything that contains a complete image of animate beings. The prayer is valid, though sinful, for one who wears it while knowing the ruling. As for incomplete images, the prohibition does not apply to them, but it is not appropriate to wear them to avoid dispute. Angels (other than the Guardian Angels) do not enter a house that contains complete images. It is disliked to pray in a place where images are hung, but the prayer is valid.