Its certainly possible that the Sister-in-Law Rachel living with Lell's is not the mother of the two children in the William Lell HH. But its just as likely that they are her children born o.o.w. and a cover-up is in place.
I looked up the death certificate of Perry Self 1872- 1949. It lists his father as Francis Self and his mother as unknown. The informant is a Pauline Russell who may be his daughter Pauline. I find it odd that the mother is not named. The father has been dead since the boy was no more than 8 but him he remembers and not the mother ? Seems fishy.
I would suggest getting the death certificate or Rachel. Its not in MO so I would check AR. The other suggestion is to track down Perry's sister Frances Self who was born ca. 1872 and check her death certificate.