Bava Basra 110a
אמר רבא הנושא אשה צריך שיבדוק באחיה
The Steipler zt"l said, " Chazal who require
to check out the brother of the girl you intend to
marry, is only applicable in a time and place where
all are Frum*. In such a case if the brother is not
Frum, then you should suspect that there is a bad root
in the Mishpacha.
In today's age where there are so many distractions
and the streets are flooded with immorality etc.
If the son ch"v goes off the Derech it is not the fault
of the father. It is not a פגם (blemish) in the family and
one may marry the sister".
With this סברא we can understand how
our Zaida Yitzchok married Rivka and wasn't
concerned with her brother Lavan. אמבצי
* דורו של משה -נחשון בן עמינדב