My alltime 2000favorite flick, and yes, it is said to be her only non-singing role.
I always suspected she got this 'mother' because she lost Gypsy to Rosalind Russell. Granted Mrs. Marcus is a poor substitute, but I really don't know who else could have managed it so amusingly. She should look out of place, but she really holds her own.
Performance wise, I think she kind of stands and waits her turn to speak (if you watch her carefully, especially at the beginning), but then so does Mickey Rooney at times, very early on.
In truth, she wasn't the 'shrewish mother-in-law'. She was justified in her resentment for son-in-law Milton Berle. He had no head for business and lost her money.
Hackett, Rooney and Winters all turned on her for no reason. They in fact were the offensive persons.
Ceasar was accidentally hit by the purse early on, so that brought on his resentment of her.
Terry-Thomas' is apparent, then she hit Silvers with that purse when he was in the hole.
Falk makes a bit of quip about her in the cab at the end.
In actuality, she was the only person making sense about dividing the money. She wanted it eight ways at the beginning.
By the time they found the money, there were six more people. So they were cheating themselves out of money from the start. They divided the money the exact same way she had said to do to begin with.

If I had to pick one person in this film, I would agree and pick Ms. Merman. If I could pick 2, I would choose Merman and Milton Berle. If I could pick 3, I'd keep the 2 and then add Terry-Thomas.

She more than held her own amongst a cast of comedic giants - t111chis is probably her best screen work, and she didn't sing a note!
"'Nature,' Mr. Allnut, is what we are put here to
rrrrrriiiiise
above!"

Yes, just watched it again.
She stole the picture and that there was a lot of competition.