Does the prayer become invalid if the praying woman's brother-in-law sees her in the prayer room or enters it?

The prayer of a woman and those with her is not invalidated by a man seeing her during prayer or by his entering the room where she is praying, whether he is a mahram (unmarriageable kin) or not. However, it is not prescribed for a non-mahram to enter upon foreign women in their private spaces unless there is a need. It is better for a woman to pray in a place that conceals her from the sight of men, especially non-mahrams.