N2
そうにない
Unlikely to, Showing no signs of
ごめん、定時で上がれそうにないから先に食べてて。
Sorry, it seems unlikely that I can leave work on time, so go ahead and eat without me.
Explanation
Attach it to the verb stem (「そうにない」 or 「そうもない」). It means 'it doesn't look like it will ...'. Judging from what you see, there is hardly any sign that it will happen in the foreseeable time: 「雨は止みそうにない」 (It doesn't look like the rain will stop). It is more vivid and firm than a plain guess like 「止まないだろう」. The polite form is 「そうにありません」.
Mixed up with
For verbs, the natural negative of 'looks like' is 「stem + そうにない」, e.g. 「出来そうにない」; to strengthen it, 「そうもない」.
Adjectives use 「stem + なさそうだ」: 「高くなさそうだ」 (doesn't look expensive), 「暇ではなさそうだ」 (doesn't look free).
「~そうではない」 sounds like a rebuttal ('it's not as it looks'); to say something seems unlikely, prefer 「そうにない」.