While I'm firmly in the age range you mention as a acceptable upper limit, (I'm 45 & my wife is 57, 12 years older than me), I also dont see an issue with the age gap between your M-I-L & her new husband. It may be a larger gap than I'd be into, but as long as they love each other & are happy together, that's what matters, not the age difference.
By the same token, 10-11 years ago, I'd have never thought of dating a woman 12 years older than me, but I'm glad I did. 10 years together so far, 9 of them married to each other, & still very happy together, & deeply in love with each other.