-I have seen some cases of 结婚公证 as you said here, but USCIS did not accept it because marriage occurred when both of your parents were alive and thus, the 结婚公证 is 公证 the fact of their marrige which has nothing to with that your mother was passed away.
您说结婚公证里“can not say your 母亲已经去世, otherwise, USCIS will not accept it”，您说那这种情况结婚公证应该怎么办呢？
-You may just ask one of your friend to translate it into English and go to the bank to notarize your friend's signature on the translation version. Your friend also need to write a statement in the translation verion that he/she knows both Chinese and English well. Then, submit both Chinese and English version.