To provide a specific answer to your question: no, there is not an established rule of etiquette for the situation you cite. The goal of etiquette, especially when throwing a party, is to put your guests at ease and allow them to have a good time. In this case, there are a few factors to consider with this specific guest:
- Your relationship with the family.
- The relationship between the two younger siblings.
- Your relationship with the other guests.
Arguments for inviting the sibling include the one you mentioned, so that there will be someone for your younger child to interact with, as well as the potential inconvenience to the primary guest's family of having to deliver their older child to the party and wait with the younger one while the younger is not invited. Especially if this is a single-parent family, which your use of the singular "parent" suggests.
But this could be a slippery slope, since there are perhaps other guests with siblings, where you know the family and the sibling less well (or in the case of schoolmates, whether you know of a sibling's existence). Stretching your hospitality and budget to the point of stress does no favors for your guests. However, if you're friendly with this particular family as a whole, and inviting this one extra guest would both make that family's life easier by keeping them from needing to find some way to entertain/get care for the younger child, as well as making your job as a host easier because you don't have to spend as much effort worrying about or responding to your younger child, I'd say that's a very polite thing to do.
If you discern that the two younger children are not close enough to enjoy being at the party together and/or you know that providing an alternative plan for the younger child presents no hardship to this family, then inviting the younger child and possibly putting them in the position of having to offer one yes and one no RSVP would be an imposition on them.
