What returns are you referring to?
There should have been a final Form 1040 for your mother for the period 1/1/2015 - 3/?/2015 (her date of death).
There should also have been a Fiduciary Return for the Estate running from her Date of Death in 3/2015 though either 12/31/2015 or if using a fiscal year, through 2/29/2016.
Whether or not this is done this way would require an analysis of the estate's income & expenses and determination if it was worthwhile for the beneficiaries to use a fiscal year to defer any income tax due until the 2016 calendar year. It gets complicated.
It would be "normal" to obtain an automatic extension for the personal Form 1040 depending upon the estate's determining the best filing alternatives as the two returns (the 1040 & 1041) are inter-related.
Is there some specific issue that you are concerned about? Extensions are automatic, don't require IRS approval, and may be necessary simplify to co-ordinate the two returns, particularly if a fiscal year is used for the estate. (The information for a fiscal year return in this case would not have been available until Mid-March at the earliest).
I wouldn't be too concerned about the extension; if you want to change the investments, make partial distributions, etc., there should be no problem doing that at this point as you are 15 months past the date of death. Further, if a trust is involved, those assets aren't subject to probate, so there should be no delay in distributing those assets unless you are dealing with an estate in excess of $5,000,000. thus requiring a Federal Estate Tax Return.
If you provide some additional information, I may be able to offer additional comments. For example, the approximate value of the estate, number of beneficiaries, etc.