We went through the same thing when we had dd2. Dd1 wasn't being naughty just difficult.
I think it was all a reaction to this new sister she had!! After having baby home for about two months she seemed to settle down back into herself, but you never know with these 3.5s could change at any time!
I just found;
including her in simple tasks for new baby helped, even though sometimes she said no. 😳
Making time, even sitting for a little exclusive time helped (I know very hard with newby).
As others have said praising her for even the smallest tasks and ignoring the bad behaviour. I mean she knows when she's over the top, but I just keep on doing something else or walk away when the tanties start.
He will come round probably just adjusting to your new bundle of joy..