My brother married a catholic girl. My husband and I did not attend the wedding. Now my family is severely upset with us How can we explain to them our position as orthodox jews, and more importantly how do I behave to my brother and his new wife?

You explain that the continuity of the Jewish People is the reason for human life and that this marriage severs that continuity. This will not make your family happy but it is the truth. Don't present it as your subjective problem as Orthodox Jews. Your relationship to your brother - how welcoming or how distant - should be determined by what is most likely to assist him in doing Teshuva. Recognizing the couple as man and wife is not however one of the options and this may lead to their maintaining a distance from you.