Answer. Before addressing your question directly, it is important to understand what state and federal law say about 55+ communities. Besides several qualifying requirements, a legally established 55+ community must have at least one person who is 55 years of age or older living in at least 80% of its occupied units.
This 80/20 rule is critical. Generally, communities strive to be over 80%, since falling below 80% means immediate disqualification. Does this mean that the 20% margin must be reserved for families with children? The answer is "No." In fact, a 55+ community may strive for 100% occupancy by persons age 55 or over. Does it mean that community management must accept otherwise qualified age 55+ applicants when the second or subsequent person occupant is 18 years of age or older? Again, the answer is "No."
If desired, a community may increase the age requirement for the second or subsequent occupant to 25 years, 30 years, or even 55+ years. Similarly, a community may impose a more restrictive minimum age requirement than 55. In order to maintain this exemption from the Fair Housing laws which prohibit discrimination against families with children, it is important for park owners and managers to make sure that all such age/occupancy requirements are properly reflected in the community's Rules and the Statement of Policy - and be consistently applied.
So, technically, assuming that your community demographic is well above the 80% floor, there is nothing per se' illegal with the situation you describe. Say, for example, the resident was not ill, but wanted her sister and child to live in the home with her, you could - but are not required - to grant permission. Under those circumstances, if the resident subsequently became ill and was hospitalized, thus leaving the sister and her child in the home on a temporary basis, I view that as an acceptable situation, and not a violation of the law. Even though the resident is not "occupying" the home while she is in the hospital, she is the lawful occupant, and would return to that home upon her recovery.
There is, metaphorically speaking, another 80/20 rule: ORS Chapter 90 is, like most laws, enacted to address essentially 80% of the most commonly observed landlord-tenant issues. That leaves the other 20% to be dealt with on an ad hoc basis, i.e. as they arise. Your situation raises issues that while not technically addressed under the law, it is close enough, and one rather simple to justify.
- It is not a violation of the state and federal law;
- It does not endanger the health, safety or welfare of others in the community; and
- Despite some grumbling from a few residents, permitting them to stay on a temporary basis is the right thing to do, even though it may not fall directly under the temporary occupant statute. (ORS 90.275)
However, in order to avoid setting some sort of "precedent" and perhaps to quell a few complaints, you should consider having a written agreement with the sister, saying something to the effect that she and her daughter are permitted to live at the home, with the understanding that she is doing so on a temporary basis and with the knowledge and consent of the hospitalized resident; that she will abide by the community rules; and that the rent will continue to be paid on time.
You should, of course, continue to monitor the situation, making sure the agreement is being honored, and that there are periodic updates on the health of the resident. If it begins to appear that the resident may not be coming back to the space, you will have to address a sale of the home to another qualifying resident.
 Your question did not address whether there is a second-person age requirement. But even if there was, and the 5-year old was in violation, my answer would be the same, i.e. this is more of a charitable response to a difficult situation. And since you are the landlord - with the power to enforce violations - you can, under these circumstances, choose not to enforce the technical violation of the second-person age limit.