He and his wives should stay n a separate house as FIL can't see the DIL s.
No BIL cant see SIL. FIL is like your own father, DIL doesn't need to wear hijab. Same thing for MIL and son-in-law. In an event of death or divorce FIL-DIL and MIL-SIL remain what is called "muhrim" never to marry relationships. Even if they get remarried to someone else, this pair still remain muhrim.
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