I want to adopt my husband's child

Even though your husband's ex-wife doesn't have physical custody of her daughter, she still retains her parental rights legally. If the daughter is staying with her for three weekends out of every month, there is no way you and your husband can claim child abandonment. I just don't see how you'll be able to adopt your stepdaughter unless her mother voluntarily relinquishes her parental rights and consents to the adoption. Has your husband spoken with her yet about it?