Married before older sibling?

My intended has a brother who is very immature for his age and relies on him heavily. A stumbling block in our announcing our engagement has been that his older brother (in his thirties, no real past relationships, and with a very negative view of married life and resentment of women) will take our engagement very personally (he says it feels like my boyfriend "doesn't love him anymore") and very badly. Some of his family feel that we should wait to marry until he has married, but we are both over 25 and his brother is in his 30's- and I honestly feel he has himself to blame for not being in a relationship and I don't ever see him holding one. How can I get my future in-laws to see this, so that the BIL is not the focal point of my marriage?