I thought it would be useful to clarify a bit more about relatives' role in someone being sectioned (being assessed under the Mental health Act) in English and Welsh law (law in different in rest of UK. I have no knowledge or ROI law).
The decision to detain someone under the Mental Health Act is made by 3 professionals. You need two doctors to write medical recommendations, and then the AMHP (a specially trained professional, usually a social worker) then makes the decision. Usually all 3 professionals asses the person together, but not always.
For Section 2 (up to 28 days) the AMHP has to inform the Nearest Relative (this is not necessarily the same person as the Next of Kin) about the decision. It is good practice to talk to people close to the patient before making the decision, but not legally necessary.
For Section 3 (up to 6 months), the AMHP has to consult the Nearest Relative before making their decision. At this point the Nearest Relative is asked if they object to the admission or not. If they don't object, relatives often feel that they are responsible for sectioning this person. This can often be a very stressful decision for the relative. But the way I explain it to Nearest Relatives, is that they shouldn't feel that weight on their shoulders, as the decision is being made by professionals.
If the Nearest Relative objects to a Section 3, the admission can't go ahead. So the Nearest Relative can actually block an admission under Section 3. In very rare cases, the Nearest Relative can be displaced by a court so the Section 3 can go ahead. This almost never happens.
Relatives involvement in Mental Health Act assessments is really important. It is often relatives that raise the alert that their family member is unwell. Relatives provide important information to the professionals to help them make the decision.
Legally, the Nearest Relative can section someone if they have two medical recommendations. I have never seen that happen though.