Not everything has to be shared. When a new sibling comes along even if they are a year old already, it can still be a shock to your older child as this new sibling is now getting into all of “their” things. You can let your older one know that they may take a few items that are really important to them and put them in their room or their side of the room on a higher shelf.
Some language I like to use:
“I know your little brother/sister is starting to play with more toys. You may take two to three toys that are really important to you and put them away”
This shows your child that you hear their concerns and respect them while at the same time showing them that there are toys that everyone may play with.