Getting married to a girl from a shia family

I have a cousin from a very Deen dar family who got involved with a girl from a Shi'ah background in the college he was studying in. She herself was "modern" and my cousin was probably able to persuade her to give up the peculiarly Shi'ah beliefs that she was brought up with. Four years later they were getting married and we were invited to their wedding. There was a wedding performed by one or their "ulama" (stating that this was a nikaah daa'imi - - not muwaqqat?!) and a reception. My father is an Aalim (a student of Mawlana Saleemullah Khan mudda zilluhu al-aali) and is also somewhat visible in the Tabligh work. Although he was not present my mother was concerned about his good name if she attended a social occasion of Shi'ah people. Also I am twenty and my brother is eighteen and my mother was concerned that we not interact socially with Shi'ah people. As a result my mother chose not to go to either occasion. Since then some family elders have come down hard on us for not coming and have said that this was Qata' rahmi and also that since the girl has accepted Islam we should have supported her but being present and should have accepted her "with open arms", and that we are running the risk of driving her away while she has accepted Islam. Should we pay heed to these objections and try to make amends? Please answer in detail.