I thought of lots of things while reading your post...
Just because he loves his sister and (thank goodness) isn't taking his frustartions out on her doesn't mean it's easy for him to share the attention she gets. Plus, she used to just lay there and now maybe she is starting to roll over and makes more fun sounds and do things that is getting lots of smiles and attention from you and dad. He may also be over the novelty of having her around-- not that he wants her gone, but he's all done with sharig the attention...
Also...
Sounds to me like he may be a) having trouble communicating his feelings {normal for his age} and b) having trouble with transition {also super normal}
For the communication thing, go to the library and ask the children's section librarian to help you pick a bunch of books on feelings for kids. Read these types of stories often with him. You can use them as reference and eventually he may be able to use them, too, and his vocabulary of feelings words will grow. Right now, he is using anger for every emotion. "Remember how FRUSTRATED that duck was when he had to wait to play with his mommy?"... "Are you feeling DISAPPOINTED like that boy in the story with the dog?" I know he's young, but it's important to let him know it's OK to be mad, frustrated, disappointed, scared, jealous, etc... and be sure to show him acceptable ways to show it. It's not OK to hit, throw things, or scream. It is OK to hug a pillow or stuffed animal, take deep breaths, squeeze a ball, etc... I worked at a school with "tough" kids who had serious reasons to walk around angry all the time (abused, neglected, ect) and each room in the school had a SAFE PLACE with things like stuffed animals to hug, small stress balls to squeeze, pillows to lay on to have some down time to chill... You may want to consider making a safe area in your living room. When he starts to freak out, take him to the area and stay with him and show him ways to calm down. Make sure he understands it's not Time Out and he is not in trouble. Eventually, you will be shocked to find him going to the safe place on his own when he feels angry. (He's young, so it may take time... but if you use it wisely he'll understand eventually what it's for and it should help)
As for the issues with transition... you said after spending one-on-one time with him he freaks out when the time is over. My son had issues with transitions, too. So I always tried to remember to qualify time with actions-- like since "five more minutes" means absolutely nothing concrete to most kids, I would say, "Put 5 more Legos on the tower and then we'll clean up", or "Pick 3 books for us to read and as soon as I've read them all I have to start making dinner so I'll put a movie on for you" (and then just before reading the 3rd book I'd be sure he remembered and understood that was the last book and our time together was about to end).