「このAちゃん、お母さんに似てる」は英語に訳すとこのようです。
A looks just like her mother!
A looks just like her mommy.
A looks exactly like her mom! (exactly → そっくり)
「お母さん」は mother/mommy/mom に相当します。Mommy だけは少し子供っぽいな言い方です。
いつもは思わないけどこの写真に限って似てるというニュアンスを加えたいならこのようです。
I usually don't think A looks like her mom, but she does in this photo.
Only in this photo, does A look anything like her mom.
Normally A doesn't look like her mom, but she does a bit in this photo.
ご参考までに。