Notice of Maintainance Allowance for all the wrong reasons

I can't recollect any statute or law that requires you to pay any such maintenance to him. There is no need to pay any such amount, unless ofcourse he holds some paperwork to indicate that he had loaned or given certain amount to you or your siblings with a promise to repay. Even then the same needs to be legally scrutinised in a court of law. At this point in time, you are under no legal obligation to pay an such amount.

Whether your father is earning?
Whether the notice sent to you is by your father or by any court/ commission?
As per your words your father was separated from you 8 yer before, then you have a good case and can save your self from paying the maintainable. Further the final opinion will depend upon seeing the facts narrated in notice.

Your father can legally claim maintenance from you if he is unable to maintain himself. Assuming that he does not have the means to sustain himself, even then the amount you have mentioned is excessive and it is unlikely that any court would direct such a large amount to be paid as maintenance. Ignore the notice and if he takes it to court then fight it out. These proceedings don't take much time to conclude.

Further, have there been any untoward advances or threats from his end to any of you? If yes, I urge you to write a complaint to the Crimes Against Women Cell in your area and have an FIR registered. Feel free to contact me if I could be of any further assistance to you.