It's so often said that there are no guarantees - but there are no guarantees in much of life. When we get married, we can't actually guarantee that we won't end up divorced, there are no guarantees that a parent won't die before a child, or that a child won't make in to adulthood.
All you can do is go with your gut feeling, then make a decision, and hope it all turns out well.
I do think that as parents, we can work hard to foster positive relationships between our children, and it is our job to do so, e.g. by teaching them how to share, be kind and patient, and to look out for each other; but there will always be cases where this just doesn't work, or of children emigrating, of family feuds in later life, and siblings genuinely not even liking each other. If you don't, I'd be working to foster those relationships, and seeing those children as my DC's brothers, where at all possible.
If not, then there is only one guarantee - that your DS will probably be the only one dealing with it when you die. By then, though, he might be married or in a loving supportive relationship, and it's unlikely that you and your DH will die at the same time anyway. So even if one of you dies young, the other parent will be there to help DC through it.