Excel Tables and Relationship

ritendersingh2 

In addition, I'd recommend to check Physical and Virtual Relationships in DAX - SQLBI

As a sample, for such tables

you may build virtual relationships using in your measures

sm TREATAS:=CALCULATE (
    SUM ( Table2[Y] ),
    TREATAS (
        VALUES ( Table1[A] ),
        Table2[B]
    )
)

//////////////////////

sm INTERSECT:=CALCULATE (
    SUM ( Table2[Y] ),
    INTERSECT (
        ALL ( Table2[B] ),
        VALUES ( Table1[A] )
    )
)

//////////////////////

sm CONTAINS:=CALCULATE (
    SUM ( Table2[Y] ),
    FILTER (
        ALL ( Table2[B] ),
        CONTAINS (
            VALUES ( Table1[A] ),
            Table1[A], Table2[B]
        )
    )
)

First one with TREATAS is most preferable but doesn't work in 2016, other two shall work.