A few years ago there was a restructure and F came into my team as a direct report.
However F had been there longer than me and had previously been in the SLT. She isn't paid as a SLT and has no direct reports. However she continues to attend the SLT meetings alongside me and I noticed today she was also added to the SLT ms team.
Other SLT members treat her as an equal. I just find it awkward and annoying that she's treated as "senior" when she's not. I do her appraisal etc. I know if it changed it people would think I was being petty.