I think DaasYochid is right.
The Mishna quoted on 17b says ?? ????? ????? (plural). But the Gemara goes on to discuss ????? ?????? (singular). The Ri is understanding the Mishna to be saying do not give it back to his children (plural) because one of the children is ????? ??? ?????? ?????? ??????. The other (living) brother, however, might be ????? ?????? ??????.
This is somewhat dochek however because the Mishna implies that both brothers were m’gayer with the father. On the other hand, this dochek should not hold us back because anyway the Gemara’s answer requires this dochek because even ????? ??? ?????? ?????? ?????? cannot be said to be literally ?? ???????? ???? ???.