I don't feel safe having my husband's 6yo son around

Sounds like a tough situation. Why not tell your husband your concerns in a calm way, and agree on a game plan - such as the two children aren't left alone together, the son goes to day camp during his stay, and you have the option of taking the baby to a hotel or out of town if you feel like there's a threat?

I hear you. When my stepkids were smaller, I would not allow the older kid (he was 11 when we got married) to be alone with me because of an incident where his brother scratched him and he told his father I had done it. No point getting mad at the kid - he was just acting out his mom's resentment of me - but there was no way I was going to put my own safety at risk again. 

That being said, this kid is 6. He is never going to be alone with the baby, because she's too little to be left alone with him. You will be there at all times. Maybe you can get some kind of family counseling that doesn't feel like counseling - like a coaching session - to ease the big kid into this situation. My prediction is that after a few days of adjustment he will be grateful for a structured and kind and calm family environment and will respond well. Give him lots of space to be alone with his dad, which is what he's afraid of losing. And above all, remember that he is just such a tiny little guy. I know he seems huge compared to your toddler but he is still tiny and deserves to be given a chance. Forget about the mom and focus your empathy on this lost kid who needs love. You don't have to fix him, you don't have to bring the hammer down, you don't have to do anything but show kindness and love while praising every little kind thing he does for his sister. 

Yes, his sister. This is his sister. He's her brother. So look for the parts of your husband you can find in him and love him like you rescued him. You can do it. He deserves to be with his dad, he deserves a shot at a normal life, and a sister could be very, very healing for him. 

Is there any discussion around removing the boy from his mother’s influence altogether?

Is this not a priority to his father?

He is young and and his behaviors and character have not been determined.  He needs a lot of support, and compassion. And probably professional help. And you could benefit from that too, in order to learn how to help him.

Its sounds like you are his best option for a mother figure.  A tremendous responsibility, and not something you signed on for. 

Your concerns of safety are important, it is difficult to tell how those issues may present. Maybe they won’t. 

An escape plan sounds good. A place to go if you feel threatened.

i do think professional advice is a good idea.  I would especially wish to avoid traumatizing this kid so that he feels locked into labeled negative behavior, and negative relationships with the healthiest adults in his world.

Try to open your heart to your husband's child. He needs his father and other caring adults in his life. In terms of practical matters, why don't you enroll him in a camp where he will have plenty of opportunity to play outdoors during the day, and then you will only have to do care taking with both children in the evenings.

My heart breaks for this child reading this.  Instead of thinking about "how to bring it up more forcefully" what about how to help this poor child and make him feel welcome and loved and wanted?  The last thing this poor kid needs is to be rejected from his father's home.  I know you don't understand as your child is younger, but 6 is still an incredibly young and fragile age.  My 6 year old acts out and misbehaves at school (and home), she sometimes hurts her younger sister (even though she adores her), and many other things 6 year olds do, especially when they're feeling sad or hurt or angry or frustrated or bored or tired or hungry...  Since I know her and love her and know her intentions are good even when her behavior isn't, I still love her and try to help her and coach her through this, because most of these behaviors represent desperate need for love and confirmation.  I know it's harder to see that when it's not your kid, but trust me, this child is still an innocent child who desperately needs love and connection.  Please welcome him and try to love him and see the good in him.  There is nothing to fear. He's 6.  He will never be alone with your child (because 6 year olds cannot be left alone and toddlers cannot be left alone) so they will always be safe and supervised.  Imagine if something happened to you, and your husband eventually remarried.  How would you want this new woman to treat your child?  I know being a step-parent can be hard but it sounds ike you've already decided this child is a problem and don't want him around, which is heart-breaking for this poor child (and his father who will now be caught between the natural desire and obligation to love and care for his child and the wishes of his wife to forego that obligation).  Also remember this is your daughter's only sibling, and siblings are a gift.  I would try to have them together as much as possible while they are growing up, for as adults they will be each other's family.

Reading through the responses so far, I disagree with the either-or framing where the choice is between being compassionate towards your stepson and protecting your baby. First of all, animal abuse is a clear warning sign that neither you nor your husband should ignore, and it sounds like your husband (probably out of a sense of guilty) is minimizing the situation.  And there's no way to supervise children 24/7 because adults need to sleep too. At the same time, your husband has a responsibility towards both of his children, and presenting things to him in a way that makes him feel like he's being forced to prioritize one over the other will just make him deaf to your concerns. 

Is there a way to address this holistically --- to talk about how to welcome your stepson and ease his transition into the household, and to say that you hope what your husband says about the child's behavior is true?  Ask yourself, if this boy was your son too, how would you handle the situation? I imagine you'd want to get him help, and also actively manage the risk of him harming his sister. Empathize with your husband's feelings and your son's, but also assert both your responsibility to keep your daughter is safe, in light of his behavior towards animals and his mom.

Finally, if I was in a situation where my child was facing a significant likelihood of injury and my partner was digging in and not being responsive to my concerns, I would find a way to remove myself and my daughter from the situation. That would be a last resort, and not an ultimatum you want to start with. But it's important to get clear with oneself about something like this.  I'm sorry you're going through this. Blended families are hard.