You're not crying because your big sister is with you today.
You don't feel alone today because you're with your older sister.
この場合、次のような言い方ができますよ。
ーYou're not crying because your big sister is with you today.
「今日はお姉ちゃんが一緒だから泣いていないんだね」
ーYou don't feel alone today because you're with your older sister.
「お姉ちゃんと一緒だから今日は寂しくないね」
ご参考まで!