No, this is another sibling. My brother died last year. His missus has just declared she is with another guy. Almost one year to the dot. It strikes me that something is slightly wrong, it's a little too fast in my opinion.
After my mum died, my dad started seeing another woman (a family friend who had separated from her husband fairly soon before, I used to play with their kids etc) pretty soon after. I don't remember the exact timeline but from what I remember she was hanging around a lot after a few months, and they were together within six months, I might be slightly out but that sort of ballpark. As a kid, it felt too quick but I imagine it doesn't matter how long passes, any new partner your dad finds is never going to replace your mum. It was a bit awkward because my dad and I never really talked about it, I think neither of us knew how to bring up the subject. One morning I went in my dad's bedroom and she was there (obviously I knew they were up to something before this). A year or two later I remember him asking how I'd feel about him remarrying (I distinctly remember I was playing a cricket management game on the Amstrad CPC at the time), he asked this in a kind of abstract way (not mentioning her name) and I got pretty emotional, in hindsight it must have been tough for him, he was kind of seeking my blessing and I couldn't give it to him. As an adult reflecting on it, I probably take a pragmatic view that if you find the 'right' person to move on to, you don't want to pass up on that opportunity just because it is 'too soon'. I mean if you fall for someone then you fall for someone right? You don't just say "ok would you mind waiting a couple of years before we take this any further?". After a bereavement you are probably vulnerable and subconsciously seeking emotional support, that can come from a new partner.
They never did marry and split after about 8-9 years, she then moved on to someone he worked with who he'd introduced her to (awkward), anyway roll forward another 20 years and they are still friends meeting up now and then (she has moved up north).