Compared to そうもない ‘very unlikely to (A)’, そうにない is a grammar structure that has a very similar meaning, but is not quite as emphasized. In this expression, も will be replaced with に. Also, apart from indicating that something is very unlikely, it may also indicate that something is showing ‘no signs of (A)’.
そうにない will connect to the ます-stem of the verb that it is indicating as being very unlikely.

• ごめん、定時（ていじ）で上（あ）がれそうにないから先（さき）に食（た）べてて。
  Sorry, it seems unlikely that I can leave work on time, so go ahead and eat without me.
• 今日（きょう）は行（い）けそうにないのでキャンセルでお願（おねが）いします。
  It is unlikely that I am going to be able to make it today, so please cancel my appointment.

Occasionally, そうにもない may also appear, and has the strongest nuance of being unlikely to happen amongst そうにない, そうもない, and そうにもない.

• 今日（きょう）は雨（あめ）が降（ふ）りそうにもないから釣（つ）りに行（い）こう！
  It is unlikely that it will rain today, so let’s go fishing!
• 列（れつ）が進（すす）みそうにもないのでまた今度（こんど）来（き）ましょう。
  The queue is showing no signs of moving so let's come back another time.

As this structure is just an extension of the use of the auxiliary verb そう (used primarily for speculation, or hearsay), it is simply indicating that (A) does not have any appearance of occuring.