x years ago, her age was a0, his age was b0.
today, her age is a1, his age is b1.

the difference between their ages is constant.
1) b1-a1 = b0-a0

their ages combined today is 91
2) a1+b1 = 91

back then (x years ago) when his age was equal to her age now
3) b0 = a1

and his age now, is equal to twice her age back then (x years ago, when b0=a1)
4) b1 = 2a0

Now we have 4 variables, and 4 equations, we can solve for all variables by substitution.

To solve for b1,
2) a1=91-b1
3) b0 = a1 = 91-b1
4) a0 = b1/2

1) b1-(91-b1)= (91-b1)-b1/2
b1-91+b1 = 91-b1-(b1/2)
3.5*b1 = 2*91
b1 = 52

to solve for a1,
2) a1 = 91-52 = 39

since they are 13 years apart, a0 and b0 are 26 and 39 respectively.