Losing faith in my marriage?

My husband's twin brother and wife are extremely controlling of him. Since before we began dating they always interfere in his life, telling him what, when, and how to do things. My husband dreads conflict and goes along with most things in order to maintain his relationships. More recently, my husband's twin made a demand on him which is inappropriate and not my husband's responsibility. His twin guilt trips him into doing things that are against most peoples better judgement, so does his wife. I have spoken to my husband about this many times. When he refused to stand up for himself, I intervened because of the stress it has put on our marriage. Nothing has changed except that I have no relationship with his brother anymore because I refuse to sit back and tolerate the abuse. My husband has demonstrated similar non-ertive behavior in other situations as well. For instance, not putting a stop to his ex-girlfriend showing up announced at our house with gifts for his family (they are not and have never been friends since they split), not standing up for me when his family has rudely implied that I have an eating disorder. The list goes on and on. He is a wonderful person whom I love deeply, I'm just not sure I can stay married to someone I am losing respect for... what kind of role model will this man be to a child?