可否幫忙看看這樣寫OK嗎？

「多謝兩位朋友在外國款待我大哥食lunch。」

可以這樣寫:
"Thank you and Mary for being so hospitable to my eldest brother, treating him to lunch in XXX."

(款待食lunch英文是treating to lunch。)
( so hospitable是那麼好客。)
(大哥是eldest brother。)
(XXX是那個外國的名稱。)

2013-07-18 18:02:52 補充：
如果盡量跟你寫的"Thank you for your hospitality for you and Mary ...."，可以改寫為:
"Thank you and Mary for your hospitality to my eldest brother, treating him to lunch when he was in XXX."

Thank for your hospitality for you and Mary had lunch with my brother

Maybe it can be this:

Thank for your hospitality of you and Mary had lunch with my brother.

如果是即時/當天答謝的話
Thank you for the hospitality of you and Mary having lunch with my brother.

假如是隔天之後答謝的話
(看來你是這情況...所以你用had以為是past, 但不是這樣表達的-食lunch已不是句子中的動詞)
Thank you for the hospitality of you and Mary to have had lunch with my brother.

以上只是按你原句的格式修正...還可以用不同的寫法

The Interpretation:-----
Thank you so much for (your) hospitality of (you and Mary) for having lunch with my (Steve's elder) brother (last weekend.)
---Hence:-Thank you so much for your hospitality of you and Mary for having lunch with my elder brother.

這樣吧
Thank you for the your hospitality of you and Mary for having lunch with my brother.

2013-07-18 15:40:45 補充：
應該是這樣
thank you for the hospitality of you and Mary for having lunch with my brother.

Thank you for your and Mary's hospitality for having lunch with my brother during his visit at xxxxx (place)

It maybe:

Thank for your hospitality for you and Mary had lunch with my brother.