You shouldn't be so bossy, especially to people who are nice enough to play with you.
You should be nice to your nieces since they are kind enough to play with you.
ーYou shouldn't be so bossy, especially to people who are nice enough to play with you.
「そんな威張らないの、特に一緒に遊んでくれてる人にはね。」
bossy で「威張り散らす」
play with ... で「…と遊ぶ」
ーYou should be nice to your nieces since they are kind enough to play with you.
「一緒に遊んでくれているんだから、姪(のお姉ちゃん達)には優しくしないと。」
be nice to ... 「…に優しくする」を使って言えると思います。
niece で「姪」
ご参考まで!