Whitney Houston's Sister-in-Law -- Thank God ... The Case Is Closed

Whitney Houston's sister-in-law Pat Houston tells TMZ, she's relieved the singer's death investigation is finally over.

Pat and Whitney's brother Gary just landed at LAX -- hours after the Beverly Hills Police Dept. concluded its investigation into Whitney's death, deciding once and for all ... foul play was NOT involved.

We asked Pat and Gary if the BHPD's findings brought them closure ... and if they were happy the case has finally been closed -- Pat said yes.