I've seen this happen a few times in my and my husband's extended trees. Based on census records, it looks like the deceased woman's unmarried sister moved in to help the widower with his children, and at some point they got married. (This was in Canada or the US.)
I've also got a case where a woman's daughter died in 1921 (aged 35) and then the woman (aged 62) married her daughter's widower (aged 44) in 1930. The man's youngest children (her grandchildren) were about 9 and 10 years old in 1930. That woman died in 1949 and the man married two more times after that (the third wife died and the fourth survived him). (This was in the US.)
That was a really confusing situation to sort out, LOL. Picture me looking at the computer screen, eyes bugging out of my head, thinking, "What? That can't be... What? Who? Nooooooo..."