I suspect you know the answer already from the way you worded the question, but no, it's not that simple. The inheritance of DNA is just too random. Depending on how many generations back you're looking at, you might in fact be descended from GirlC and not share any DNA with her descendants, while you do share some DNA with descendants of BoyA that goes back to HusbandX and WifeX.

What you could do is follow the collateral lines. If GirlC is married to SpouseY, and you match some descendants of SpouseY's siblings, that would be point in favor of GirlC.