The correct translation is "我正打算去我妹妹家玩"。
"i am going to "means "我正打算去"、 "我正准备去".The translation“我正去”means nothing to Chinese.The translation"我正要去"and"我要去"are ok.
"my sister's house" means"我妹妹的家" which always replaced by "我妹妹家" in Chinese.
We always put the adverbial "now(现在)" at the beginning of the Chinese sentence.