Discuss with them their strengths and the specific needs of the care receiver. Everyone needs to be realistic. If they are now willing to help, please accept it. But if they've been out of the loop for a long time, figure out why and then agree on specific tasks each does best (such as record keeping, trips to the doctor, grocery shopping or just spelling you from time to time).
Good luck. This will take some communication but the end result could be well worth it.
Is find that a difficult question to answer. You have to wonder why they weren't there in the past. Do you really want their help? I have siblings that do nothing and I did everything except beg for some help and got a No too many excuses basically they are just too busy ( like i'm not) so don't know what to say to you. The part that they " want to be in charge" is strange to me. I would think long and hard before I'd except help now, that's just my opinion though, good luck to you.
Good luck. This will take some communication but the end result could be well worth it.
Carol