Here are your parts
Replacement parts for WHIRLPOOL RBD275PDB14 27`` BUILT-IN ELECTRIC DOUBLE OVEN STD-CLEAN LOWER SELF-CLEAN UPPER | AppliancePartsPros.com
See the attachment for the tech sheet.
Since the control does beep I would say that it's power supply fuse is OK.
Could be a problem with the power transformer. The tech sheet gives the winding resistances. Be sure to also check between the windings and the case just to make sure a winding has not shorted out internally to each other or the case.
Unfortunately I do not know what voltage the transformer should put out.
Perhaps someone else will jump in who does.
My question is that this control panel also comes with a small sister board, what is this for?
Part of the latch system, see the tech sheet.
how can I be sure that the control panel is truely the problem. according to your parts list,
All you can do is eliminate other cause, if everything else looks OK then all that is left is the control board. No manufacturer publishes board schematics so you cannot troubleshoot the board itself.
as well as Whirlpool's they no longer manufacture this panel and have no replacements, WHY?
You would have to ask Whirlpool why the board is obsolete.
There are companies out there that will repair blown timers/control boards.
Just Google " appliance timer repair"
Next question: After reading all these forum questions and answers on whirlpool oven problems, why hasn't there been a consumer recall on their control panels.
Just about all parts fail eventually and unless it is a very, very common problem and there is a customer uproar or it is a safety issue you will not see a recall.