Non-PRO Japanese to English Art/Literary Cinema, Film, TV, Drama

Dear ProZ members,

I'm not understanding what 横取りする means in the following sentence from a cartoon. The main characters are a group of elementary school students, and one of them is on bad terms with his older sister. When she suddenly appears where the kids are playing, he gets angry and shouts:

やなんだよ。おれが遊んでいるとさ。いっつも、横取りしてさ。どうせ、人に自慢したいだけじゃん！友達多い、とかいってさ
(I don't like this. You always butt in when I'm playing. You just want to brag with everyone about the many friends you have.)

The literal meaning ("to steal") doesn't seem to fit, the kid just seems to be disturbed by the presence of his sister. Unless he's implying that she came to steal his friends, but given that she always talk badly about them I don't think that's the case.

Any hints?

Thank you so much!