Philip is correct about everything in his reply with the possible exception of published works. Generally speaking, it is quite possible to get the copyrights back from publishers as long as the composer was careful about reviewing the publication contracts signed. There are two sections of the US copyright law, 203 and 304(c), which deal with the termination of transfers and licenses granted by the author or copyright owner. For works first published under the copyright owner's authority 1978 and later, section 203 stipulates that a termination must be initiated no earlier than 35 years from the date of transfer, but not later than 40 years from the date of transfer. For works published before 1978, section 304(c) states that termination proceedings must be initiated no earlier than 56 years after the original date of transfer, but no later than 61 years from date of transfer, or starting at 75 years but not later than 80 years from the date of transfer. For all practical purposes, the date of transfer is the same as the date of publication (usually).
Assuming everything published before 1964 was duly renewed 28 years after publication, the published works are protected in the USA for 95 years from publication. Items first published after 1977 are protected until January 1, 2071 in the USA and EU, January 1, 2051 in Canada and most of Asia. Should your sister agree, the posting of your mother's work on IMSLP will not alter their copyright status or ownership-split in any way. I recommend you have a look at Leo Ornstein's category, which has been made possible thanks to the blessing of his son Severo Ornstein, to see how a composer whose work is under copyright appears on the wiki. If your sister is amenable, I recommend that you select the non-commercial, no derivtaives version of the Creative Commons license as this will allow for performance and broadcast royalties to continue to be collected by ASCAP or BMI for all but unpaid benefit performances (which they can't collect on in any case legally).
Carolus (IMSLP Copyright Admin)
PS: Posting on IMSLP would most likely meet the legal definition of "publication" under US law, although it would appear (after perusing your webiste) that the offer of scores "Copies of scores may be obtained by contacting firstname.lastname@example.org
." would likewise meet the criteria. A quick search of OCLC generated about 121 items (numerous duplicates, reissues, listings of copies of manuscripts, etc.) - which probably boils down to about 30 works altogether being published. The earliest item I saw was issued in 1933 in Henry Cowell's periodical New Music
. Apparently a number of things were self-published(?) under the imprint "Catamount Facsimile Editions" also. The vast majority of works would fall under the life-plus-70 calculation of copyright term.