Attn STEPMOTHERS..... Help!

I'm seeking advice from other step moms.... I have a 6 year old step daughter. I have been in her life since she was 1. She lives 8 hours away from us, so now that she has started school, we only have custody of her in blocks (holidays, summer, spring break, etc) prior to that it was 50/50. Our relationship has always been a bit rocky, because my husband is the epitome of the "Disneyland Dad." She never gets in trouble or is punished for ANYTHING when we have her, there are no rules or bedtime because as he puts it "she's only 6, and I want her to look forward to coming here!" I DO understand that, but I am currently 35 weeks pregnant with our first child, and I feel like rules and structure need to be the same across the board for the sake of both kids! Anyhow, last night my husband was talking to her on the phone, and she wanted to talk to me. I asked her how her Easter was, and she completely ignores the question, and says "I hope you don't have a 'go away carriage.'" Of course I had no idea what she was talking about, so she says "I hope your baby doesn't die in your belly!" Her mother didn't say a word. I quickly tried not to show my shock and told her how healthy the baby and I are. My husbands only response was "she's only 6, she doesn't know what she's saying!" First, at what age do you start holding children responsible for their rude comments instead of making excuses? I just feel that it should have been addressed with her, maybe explained what that meant. And second where would a 6 year old even come up with that?!? Is it from her mother?? Am I wrong for being upset?!?