Older sibling 5 years between

I had a 5 year old and a 4 month old. Since my 4 month old was born my 5 year old has been acting out. At first we were not making a big deal about it bc we thought she'd grow out of it quick but its been 4 months and its still going on. We both take turns watching the baby so we can spend 1 on 1 time with our oldest but as soon as we are done spending time with her she's back to her sour mean attitude. We've tried time outs and everything bit i don't see any improvement at all. Anyone else have a tough time going from 1 child to two?? Does it get better? What did you to do br help the situation??