The sentence is most expressing an matter fact, meanings such as "I have no choice because..", or "It's all because...", but not in a sad and helpless way but usually an expression used between friends or closed ones for fondness. I would translate the sentences you mentioned as: 谁叫我是你妹妹：('A' asked for a favour and 'B' agreeed and said: I'm your sister after all. 谁叫我喜欢你：It's all because I like you so much. I agree the tone's not entirely the same, but don't know how to do a better translation...

这个很难说．．．这是个很＂中文的＂说法．　一般不会这么说． 但你非要说不可： "谁叫我是你妹妹” Who said I was your sister? “谁叫我喜欢你” Who said I should like you? 也不是很标准，　要看你的态度才能翻译更正确一点 我建议你不要这么说（用英语的时候）还是找别的说法表达你的意思。　

because i am your sister, because i love you,这样就可以了 中译英的话，最好意译