Step kids are still my kids but what if they get upset?

Here we go, my husband has 2 boys from his previous marriage. They are 7 and 10. We have them every other weekend and on wednesdays plus summer and breaks ect. Their mother is not a nice woman what so ever... she is quite the nasty woman and uses the boys as leverage. Since I have been involved my other half and her have not spoken in 4 years and only in court or send cold texts about pick up / drop off times ect. They had a terrible custody battle where she tore thoes boys all the way through. Its Quite odd as they had kids and never say a word to one another only follow the parenting plan. On another note I love those boys to death. Although clearly their mother hates me she freaks out on them about their dad and I having a baby.... because that is going on they are not the least bit excited about a sibling. And say its not their sibling as I am not their mom and their mom hates me? Its complete bull... ever since we told them its been nothing but hell. I have never forced motherhood on them, they call me what they want. I am simply another human I am not their mom and I continuously make sure they love their mom and feel free to talk any time they want. I even help with mothers day stuff and make sure they treat her with respect and never talk bad about her. I am not the new girl that hates the ex. I prefer to embrace the past and to make sure the kids dont have any stress over it. I have never said a derogatory thing about their mom and never plan to. they say She has been yelling at them non stop about this new baby, telling them to hate me and one time told the youngest to never come near me unless its to punch me? Telling them it its not their sibling and they need to hate the baby. This has caused them to literally want nothing to do with the process. I asked if they wanted to go to the ultrasound to see and they said no because their mom would yell at them and send them to their room? I can feel they are starting to resent the unborn. I am trying to make them involved but they want nothing to do with it?

On another note this is my first child. Of course I will be over joyed. First off The boys prefer me to treat them like kids and not act like a mom. Okay whatever you want I am here to be your friend not enemy but do you think they will get jealous of the baby when it comes? Im not saying I will treat my children any different but we all know that parents with biological children seem to have a bond different than inherited children. I am completely confused and have no way how to handle this. I am 21 and due in march. I have has no kids except the 2 boys that have stolen my heart.