elder stepsister
Refers to an older female step-sibling, specifically one who is older than the speaker. The term is not commonly used in modern Japanese; 義理の姉 (ぎりのあね) is more standard.
She is my elder stepsister.
My elder stepsister got married last year.
Compound of 継 (まま, 'step-') and 姉 (ねえ, 'elder sister'). The reading まま is an old prefix for step-relationships, as in 継母 (ままはは, 'stepmother').