That would be her decision. The challenge is whether you can put up with it. I'd guess the big question is whether you really think the relationship can go a long way if it works out between you and her . . . You need to be able to accept both outcomes, either the happy ending or the not so happy ending. If you think you can do that (well, i'd say either way you'd have a 99% chance to say yes wtihout much thought).
If so, then there nothing wrong with it, just make sure you don't trigger any inappropriate moves, though by knowing that she's already engaged, I personally feel that its a serious 'honour' issue there . . Which is why, its best you don't even try to suggest anything to her about it . .Just one of the principles of chilvary I guess . .
If you don't mind the wait, and if you don't mind losing everything you invested over the wait, then I guess its fine, Otherwise, an absolute no, because a lot of guys go crazy when they u-know-what.
Key of reference: If you really do love her ('like' doesn't equate to anything under any circumstances), you will do eveyrthing you can to ensure her happiness in life. You know what I mean. . .
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The key to victory is the element of surprise.
When that doesn't work, I guess I just gotta gun you down.