Question 1070479
x, the youngest sister;


{{{x+x+1+x+2=207}}}

{{{3x+3=207}}}


{{{3x=204}}}


{{{highlight_green(x=68)}}}


Their ages:  68, 69, 70.
Oldest sister is 70 years.




-
ALTERNATIVE
The question asks for the age of the Oldest sister.
Let x be age of the oldest sister.
{{{(x-2)+(x-1)+x=207}}}
{{{x-2+x-1+x=207}}}
{{{3x-3=207}}}
{{{3x=210}}}
{{{highlight(highlight(x=70))}}}