Translation:Those are my mother's old clothes.
"Those are my Mom's old clothes." has been reported and should be accepted.
母亲 means mother, 妈妈 means mom. The correct translation should be "mom's clothes"
I suppose asking it to accept the British spelling ("mum") is too much... I cannot bring myself to use American spellings.
I feel your pain. A few lessons back, every time I had to type "steamed pork dumplings" instead of "soup dumplings," I died a little bit inside.