Will probate

1.  IF will is duly probated, THEN the will obtains "finality", more so since there is always a chance that there may surface another will, at a later date, and thus fresh never ending disputes start arising, at the hands of known /unknown legal heirs /beneficiary/s.

2.  The mentioned "no objection" presently at the hands of sisters, may be withdrawn anytime in future and disputes may be filed by the trusted ones, thus junking down the non-probated will.

Keep Smiling .... Hemant Agarwal
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If one were to rely on will in Mumbai one must obtain probate.

You may avoid it in the following way and it won't be illegal also.

However as regards the flat in Mumbai if it is in a registered cooperative housing society get the share certificate transferred in your names, make a release deed in which the sisters release their respective right title interest in the flat to the brother and register the deed" the releasee will get absolute right title and interest in the flat. 

Alternately your sisters could gift their shares to you and register it. Stamp duty and registration would be minimum. For release deed the registration fee is 1000 rupees and and stamp. Duty is 500 rupees in case of gift deed it s 500 stamp duty and maximum 30000 the registration fee. 

Probate of a Will in Mumbai is a must. I also agree with Shri Hemant Agarwal regarding "Finality of Will"

I agree that Probate is imperative and one should not be tempted to go for short-cuts which may invite future issues.

expenses will be minimal. Contact a practicing advocate