我不再能了...

- 你好爸妈，这是我的女朋友，她不吸烟也不喝酒。
- 真的？你真的不吸烟也不喝酒吗？
- 我...我...我再也不能了...

- Hi Mom and Dad, it is my girlfriend. She doesn't smoke and drink.
- Really? You really don't smoke and drink?
- I...I...I can't more...

我不再能了...

- 你好爸妈，这是我的女朋友，她不吸烟也不喝酒。
- 真的嗎？你真得誡了的不吸烟也不喝酒吗？
- 我...我...我不再不能近烟酒了...

I could not understand the gist of the joke in English.  However, looking at your Chinese...  I was able to get it.  Hence...

- Hi Mom and Dad, it this is my girlfriend. She doesn't smoke and drink.
- Really? You really don't smoke and drink anymore?
- I...I...I Can't anymore! ...