It's partly because it'd be awkward, but if it doesn't work out between then and their parents stay together then she's kinda forced to continue seeing him. If that does happen it could be really awkward and make either of them miserable.
If your mom and her boyfriend are serious at all then you should probably not go after her boyfriends son. I guess it just depends on if they're cool with it. Just make sure to be yourself and flirt and if he likes you it should work.
