] Lucy says he talked constantly in their praise, but I feel that it might have been the opposite: that he said very little about them, gripped by his sense of guilt, and that she picked up on this reticence.

Lucy is probably exaggerating, but I can't imagine that Edward was that silent - if he only mentioned his sisters-in-law once and then never again, that's hardly enough information to leap to the conclusion that he's acquired a fondness for one of them. However, it's also very difficult to see Edward babbling apace about the wonderful Elinor and how much he enjoyed her company.

My guess is that Lucy, always alert to any potential threat on the horizon, winkled the information out of him by intense questioning. They've just been separated for a long stretch of time, and since that's common for them this probably isn't the first time she's given him the third degree after a long separation. He mentioned sisters-in-law, so probably her first question after that was to ask how many, how old, etc. Upon determining that the two eldest were 19 and 17 (very eligible ages for snatching away dear Edward) she probed further, asking about their "tastes, habits and pursuits" :) - basically forced him to talk about it until she saw that talking about Elinor was getting a definite reaction - probably one of, as you said, getting uncomfortable and really not wanting to talk further. There's no way Edward would rhapsodize about Elinor in Lucy's presence, but he would probably get defensive very quickly after saying the usual things about how she was charming, a good painter, etc.