Visual or branding guidelines is a broad term that could describe any number of things; the customary ambiguity of GD terms; yet I've noticed that in most cases they are a necessary measure to ensure third parties apply your branding correctly.
What you're talking about is more a consistent similarity and feel between the apps which I think can be better categorised as creating a theme or template.
A uniform feel across digital content is often best achieved through consistency of layout elements positioning and behaviour as is evident in the Stack Exchange network of sites.
Without knowing what your apps are about or how un/connected they are to each other, I would advise you to focus on creating the consistent layout elements that will exist in all of the apps.
Further to this, I think that the fewer consistent elements you have, the more you should try to make the ones that you do have stand out.
For example, if the only consistent element is the navigation, make that navigation memorable and a visual focal point.
On the flip side, if you have 8 consistent elements you can relax and tone it down a bit, as simply using the same layout will go a long way towards achieving a similar feel.