At this stage I wouldn't mention anything about bf's. The reason I say this is that there was a member (or the gf of a member) on this forum who applied for a family visit visa to stay with her aunt which was refused. She later applied for a fiancee visa and she was refused again because she hadn't said anything about a bf or fiancee in the UK when she applied for the family visit visa.
As you have already had family visit visa(s) and have never told them about a bf on your previous applications, it's quite likely that if you tell them this when you apply for a family visit visa again, they may refuse because you never mentioned it on your previous applications.
I agree with the principle behind Darren's advice, that honesty is the best policy, but at this stage I think that telling them this is likely to lead to a refusal if you apply again for a family visit visa.
With regard to any future fiancee visa application, as long as you don't supply evidence of a relationship over a period which covers most of your visits so far, then I don't think the ECO will have grounds to refuse you a fiancee visa.
To avoid this sort of problem the scenario must be something like this, you went to the UK to visit your brother, while you were there you met a man, after a short time you got engaged, so at the end of your family visit you went home and applied for a fiancee visa.
Iain.