I think points of law will say that a deed IS LEGAL without being recorded, if it is prepared properly. This point of having in recorded is to get it in line so that other persons don’t file liens or claims ahead of it.
My understanding of deeds, is that only the Grantor(s) are required to sign and notarize the deed. And, in fact, the receiving party(s), the grantees, do NOT have to have signatures on the deeds.
So I don’t understand the point of you and your sister being “present to sign the deed”.
I don’t know if an attorney can himself authenticate a signature like a notary would, someone else will have to answer that. If this attorney is hired by your mother, I suppose it is possible they may tear up the deed if it hasn’t been recorded. It certainly is advisable, any time you’re receiving a deed, to have it in your little hot hands and personally take it to the Recorder’s office to have it recorded.