I have been asked to check if the affiliated group rules apply to this scenario. Any assistance would be greatly appreciated.
Corp A is owned 100% by Dad.
Corp B is owned 49% by Son and 51% by Daughter-in-Law. Currently, only Corp A has a profit sharing plan, and the question is if the employees of Corp B must be allowed in to Corp A's PS plan.

Need more details, because you need to check the attribution rules under IRC 318 (I believe). Sounds like a controlled group to me. What about doing a QSLOB?

Affiliated Service Groups and Controlled Groups are different animals. Attribution (as noted above) will be key to whether there's a controlled group. There's a rule somewhere in 1563 about attribution within a company doesn't necessarily carry outside it (don't have my books with me). This my bear some fruit.

ASG rules require at least one company being an FSO or A-ORG. Check the Code to see if any qualify as either. (It's always good to keep the Code open when looking at these rules... mucho confusing).

We all might be able to help with more info.