NTA.

He moved on very quickly. Anyone with empathy could see how that would be hard for young children. It seems very fast for him, after losing a spouse.

Honestly, even in a hypothetical completely neutral situation (unmarried, no kids), I'd worry about a friend who married someone a little over a year after meeting them.

But in my mind the speed of the remarriage isn't the main problem - it's that he didn't take your feelings into account then, or at any point in between, and he still isn't now. You tried to talk to him like an adult about this, and he responded by making it all about his hurt feelings while diminishing your and your siblings' feelings.

He's just hitting the point where all his children are grown and gone. Instead of making a small effort to make things right, he's doubling down on this behavior. This is how parents end up ignored by their adult children.