For some reason my daughter acts up more around my niece and nephew?

My brother and his wife just moved in temporarily with my mom. She livesabout 10 min from me, and we are thrilled to have her cousins enarby. They are 2 and 4, my daughter is 3. My nephew is the youngest. I have noticed when we go to aprks with them like today for example my daughter will exclude or try to exclude other kdis and act like the slide is "only for her and her cousin" my niece...she will also exclude my nephew.

She's a;lways territorial but acts it more so with them around...almsot like she's showing off.

Also she gets really jealous of any attention I give my 2 y/o nephew...and she acts jealous of him period but not my niece. If I play with him or pick him up she will run and say "my mommy" and try to grab me away. I try to tell her that he's her cousin and she needs to be nice to him and that I love him. But it just doesn't sink in with her. She just keeps being thatw ay toward him. Soemtimes he will cry even I feel so bad.

How can I get her to be ncier to my nephew and give him attention while letting her know that I can love another child but she'll always be my #1 ?