Sibling jealousy/posessiveness

I’m not a parent myself yet but I’m very close to a certain friend and her younger children, specifically her 3.5 year old son. He’s a very happy, friendly kid and has become very attached to me. He has a younger brother who just turned 2 and is learning to talk (can’t say a ton yet, but understands a lot).

Now that the 2 year old is becoming verbal, he is connecting with me more and we’ve been getting closer. He’s more outwardly affectionate and likes to give hugs. The problem is that the 3.5 year old gets quite jealous, pushing his brother out of the way and wrapping his arms around my legs. He refers to me as “my Yorkie” (username for privacy).

How should I react in the moment? This is a family of 10 so the kids are used to sharing, but these two are the youngest.