] When he first saw Marianne when she was playing the piano, he was merely respectful of her playing performance. (Leave it to Mrs. Jennings to jump at conclusions.) It was in Chapter 10 that he really show the attachment, long after Mrs. Jennings and Sir John stopped teasing him about it. Elinor, however, noticed it at that point.

I had the impression that Colonel Brandon's respect for Marianne's playing was the 'spark' that started the flame. The reason that he didn't show his attachment was because Mrs. Jennings and Sir John teased him so much, which is very uncomfortable when you do actually like someone. With the end of the teasing, Colonel Brandon was able to show his true affections, when everyone else was beyond interest in it. Only Elinor, who is very observable, noticed the Colonel's interest.

] I wondered why Sir John believed it for awhile, did he simply believed his mother-in-law's assertion?

I agree, but I also think that maybe since Sir John had known the Colonel for a long time, that Sir John had some kind of insight to the Colonel's interests. Then again, Sir John cares more for follies than serious discussions. :)