I had my daughter 3 days ago but my son is being jealous towards her with me. He won't let my mother do anything with him and I have been ordered not to pick anything up over 10lbs for a week or 2. Last night, he cried with my mom for a good 20 minutes while she tried to rock him but he only calmed down until after I took him over.
Is this your first day home with both of them? I mean u can't really expect him to be ok with it the first day ofter 15 of it just being him. No one can really tell u how long it will take, but hopefully not long, just make sure u make time for just him and u.
When he gets used to the routine of having a new baby in the house, and when he realizes that he is still just as loved. It's hard with a 15 month old because at that age, kids can't express why they're frustrated, only that they are.
Try including him in some of the little things you do for the new baby. Give him a few extra cuddles when you can. Good luck, and soon, you'll see just what an amazing big brother he will be for his sister.