Wrapping: Sallie is 60 ---that means she was born about 1840; it also means that she could have been having children since 1855, but she didn't have San til she was 36 in abt 1876.
And it stands to reason that Sallie's mother could have had children from about 14-15 years of age into her forties too (which could be this uncle & aunt); Sallie's dad could have sired children from an early teenager up to when he passed away (which could be these two also - same mother as Sallie's or a new wife) ...could be a span of 50-60 years for a man! The same holds true for San's FATHER's mother & father. My mom was the youngest of ten with a 21-year span to the oldest. Therefore, this just not an unusual situation.
An easier way of putting it, is that this uncle & aunt to San, are Sallie's brother & sister, either full or 1/2, OR Sallie's brother-in-law & sister-in-law, again either full or 1/2 to San's father.
To know the exact relationship ( San's mother's or father's side of the family), you'd have to do your tree / branches. If you need help with that, can help you.