it won't be like [...] the sudden romance recontextualizes the whole story
Dunno. Let's say your dad died, and some time after you walk in on your mom making out with his sister. I would argue that even if they got together and started banging after the funeral, it'd still recontextualize not only your parents marriage, but also your whole personal life story up till this point. #nowwhat
There's always an ick factor present when widow or widower gets involved romantically or sexually with family member on side of the deceased. Square or cube that when it also involves changing teams.
...I guess it's a good thing that the brother kicked the bucket so soon.