minimum participation-2 person rule

An individual is a sole proprietor and also is a partner in a partnership with 51% ownership and one other partner at 49%. I think this is a brother sister controlled group. Does this mean that the minority partner must be covered in the defined benefit plan of the sole proprietor to satisfy the 2 participant floor of 401(a)(26) considering the controlled group? If so, could a solution be for the  spouse to become  a W-2 employee of the sole proprietor and then cover the spouse to satisfy 401(a)(26)?

A brother-sister controlled group requires 80% or more common control. It doesn't sound like that exists here.

Yes, there is no control group, but if each entity had the same type of plan then the 100%/51% owner would have an aggregated 415 limit.

Isn’t that only if it’s a parent-subsidiary? Not brother-sister?

No CG but watch out for potential ASG issues if businesses are "overlapping".  Yes, employing a spouse would solve 401(a)(26).

John...do you think it is a brother sister controlled group?...I am not sure why it would not be; 80% control of both entities and a controlling interest of both entities when considering identical ownership..

A parent-subsidiary is when 80% or more of one company is owned by the parent company. Owned by company. If they have common shareholders, that’s is a brother-sister scenario.

Brother sister controlled group has two requirements:  1) Common ownership of 80% or more of each entity by five or fewer individuals and 2) the same five or fewer individuals have identical ownership of more than 50%  (See 26 CFR § 1.1563-1 - Definition of controlled group of corporations and component members and related concepts.)