What about the Shwigger?

אחות נחשון מכאן למדנו- צריך לבדוק באחיה According to the ספר חסידים שע"ו , you should check out not only her brother but also her mother. (Shwigger) In 'שולחן ערוך אהע"ז נ':ה  it says that if one found out that the girl's sister converted he may back out of the Shidduch without any penalties. (seems good idea to check out  the sisters as well) The שו"ת חתם סופר אהע"ז ח"א :קי"ג  there is no need to be concerned with the parents sibling.   The Steipler zt"l held, that the Chiyuv of לבדוק באחיה was at a time and place where all are Frum*. In such a case if the brother is not  Frum, then we should suspect that there is a bad root in the Mishpacha. In today's age where there are so many other distractions and the streets are flooded with immorality etc. If a son ch"v  goes off the Derech it is not the fault of the parents. It is not a פגם (blemish) in the family and  one may marry his sister".