What is the ruling on the prayer of one who was separated from his two brothers in the congregational row due to an invalid prayer of a person between them?

The statement "his prayer is not valid" can mean two things:

1. If the worshiper is an adult and his prayer becomes invalid after he has started it (such as by breaking his ablution or remembering an impurity), he is not permitted to remain in the prayer and must exit. His exiting or remaining does not affect the validity of the prayer of the other congregants, because straightening the rows and filling gaps is a Sunnah and not an obligation according to the majority of scholars.

2. If a child is intended, then if the child is discerning, his prayer is valid. If the child is not discerning, he should not stand in the row to avoid breaking the row, but if he is placed there, it does not affect the validity of the prayer of the rest of the congregants.

Generally speaking, the prayer of a congregant is not invalidated by the invalidation of the prayer of any other congregant, even if all their prayers become invalid.