Trying my hardest not to get upset

So I’m currently 37 weeks pregnant, it’s my first biological child. My husband has 2 sons from a previous marriage 12 and 10. I’ve been with my husband for 5 years now so I’ve been there for a big part of the boys lives and I love them like they’re my own. Their mom and her husband have a daughter together and their mom is also having another child in December. They’re such awesome big brothers to their younger sister and it’s heart warming how attached they are to her. But they seem really disappointed that I’m pregnant, I don’t know if it’s because the age they’re at, or because they had a baby free and thus stress free house when they come to stay with us and now that’s changing. I don’t know if it’s because I’m having a boy and their mom is too so they’re getting 2 brothers a month apart from each other... anyway they just seem super opposed to having a baby brother. We don’t get to see them that often because they live in a separate state and I’m really trying to not let it hurt my feelings. But I’m really worried they aren’t going to bond with their brother. It’s hard because they’re so sweet about their sister from their mom. Has anyone else experienced something like this? I need advice and I definitely don’t want them to know that it bothers me if they don’t like/bond with my biological son.... I just really want them to.