3 year old has started hitting the baby. Please tell me this is a phase

Aarav was so excited about the baby for the first month. Now, whenever I'm feeding her, he comes and hits her head or squeezes her leg and looks at me to see what I'll do. I've tried the calm voice, the "hands are for gentle touches" line, sitting him next to me with a book. My mother-in-law's approach is a loud "NAHI" which makes him cry and then he does it again in an hour. I know he's jealous, I know he's three, I know it's normal. I just want to hear from someone whose kids came out of this phase alive and still liking each other.