Well, biologically related or not doesn't make a difference socially speaking when you are dealing with family. Telling them usually doesn't lead to anything positive, unless your first goal is being up front with them. Even if they seem to react positively at first, over time their wandering minds are going to get the best of them as they watch you get along and become more closely attached. Sometimes they are actual concerns, other times you become a convenient target when their own relationship with their kid hits a rough patch.
Most of all, it's harder to extract yourself for the sake of your safety, her well-being, or just to eliminate some bad drama from your life. On the other hand, family settings again... often the easiest settings to fall in love. Lots of trust by others, lots of excuses to see one another, and lots of prepackaged shared parts of your lives to talk about and connect to one another with. Think about it next time. Until then, good luck.