How did you make sure your step kids dont feel replaced

Chelsea

I have two step kids with my husband.. a ten year old girl and 8 year old boy and I am currently expecting a baby boy in March. At my husbands ex wife's house they have two younger boy siblings that they have already had to adjust to. They have expressed to us many times that they feel that they come last to their siblings at their moms and I know it hurts their feelings. The little girl does okay with it, she is very mature and it helps that she is the only girl and so that plays a role in making her continue to feel special. However his son has had a lot of trouble with it.. he has had some behavior issues and lashing out that we feel like stems from feeling like he is "less loved" or gets a lack of attention. I can tell he isn't particularly excited for his dad to have another boy.. especially since him and has dad have a special bond and I think he really counts on that. Obviously this is my first and probably only biological child I will have so I want to relish in the joy and excitement and I dont want to have to temper it.. but also feel guilty. I want to find a happy balance of making his kids still feel super loved and appreciated in our home, but also can tell I may get my feelings hurt because I can tend to be a bit sensitive... any advice or scenarios you have dealt with I would love to hear!