@whiteopal,
Of course, if your boyfriend ever found out that you had fooled around (to whatever extent you are talking about, which isn't really clear) with his brother, chances are high that he would probably leave you...most brothers feel that way about such things (but not all).
But the above isn't a matter of whether or not you deserve your boyfriend.
As for feeling dirty....you're young enough not to know that what you've done now will be nothing compared to what you've done 20 years from now...humans are sexual beings. You'll also learn that there is a very wide variety of people who enjoy sexual things much different to what you enjoy, or would have thought you enjoyed...there are people who will do things you will consider slutty, or wrong, or perverted (or whatever)...and yet, you won't know they do these things, and you will like them for who they are (ie. a persons sexual taste is usually meaningless to whether or not they are nice people, or not). So I wouldn't put too much meaning on feeling dirty.