I think as always it's an expectation that any new person would be a stranger taking people away. They probably don't think "hey, one more person to show up for my games if the others can't!" yet it would happen if they develop a close bond over time. No, to them it's one person to take their parents away from taking care of them, not one more person to help with stuff.
Yet with three people, one can imagine there could be one there helping while the two others are together. Even more so with more people obviously. That could mean letting their parents spend more time together from time to time, as well.
But I'm not surprised at all. Especially since you said they're boys, and well, when I grew up boys that age had that weird thing with their mothers, even suggesting they had sex with their dads was like an insult, because they were their moms, and they were pure and holy or something. So having a relationship with someone else? Inconceivable for them.