UPDATE - K7AHR decided to check the ground conditions for these two peaks. Both are apparently accessible with the usual cautions regarding hiking in burn zones.
Hearing that the Big Hollow TH is again open, I began making plans to activate Timbered Peak and Sister Rocks (W7W/LC-064). Unfortunately, a BARC map (dated September 30, 2020) indicates that Timbered Peak was severely damaged in the Big Hollow fire. At this time, there appears to be no way to safely access either summit. More below...
After reviewing maps and other info, I spoke to a ranger at the Mt Adams station. FR58 has been logged and is clear and the Big Hollow and Observation Peak THs are open. That said, trails and side roads have not been cleared of downed trees or repaired. Timbered Peak was definitely burned over, as was the Observation Peak trail (which is used to access Sister Rocks). The ranger has not been in the Sister Rocks area but thought it had burned over, too.
I will update this note after my next trip to the area but for now it is safe to assume that access to either summit is hazardous at best, dangerous at worst. Trails may be hard to impossible to follow and traveling without map/compass/GPS is ill-advised.
You can find a copy of the BARC map here.