As is often the case, there's no 1 to 1 translation of about in French.
Environ only translates as about when it marks an approximate quantity:
She was about ten = Elle avait dix ans environ // Elle avait environ dix ans
When about is an adverb, it translates as an adverb or an adverbial phrase in French too:
I was about to leave = J'étais sur le point de partir
When about is a preposition marking the topic of a conversation, a speech, a book, or the like, there are three main options:
de: It only has this meaning with a few verbs, but this includes a very important one: parler (also discuter and causer, all essentially meaning speak about). Your sentence could thus translate to:
Je voudrais vous parler de ma sœur - I'd like to speak to you about my sister
sur: This is the general preposition that's used with most other verbs (and it can be used with parler too, although with a different meaning in some dialects)
J'aurais aimé vous dire quelque mots sur ma sœur - I'd have liked to tell you a few words about my sister
à propos (de): unlike the other two (the most and fifth most used prepositions in French), it's unambiguous and can only mean about. It's however quite a bit clunkier, clocking in at four syllables and is mostly thus used for maximum clarity or in verb-less sentence fragments:
J'aurais aimé vous dire quelque mots à propos de ma sœur
Even then it has a meaning that's not shared by English's about:
À propos de ta sœur, elle se l'est enfin payée, sa liseuse Kindle ? - Speaking of your sister, did she finally buy herself a Kindle?
À propos can also mean by the way.