Quote from VegaTDM »Communication is key, in both casual and competitive play. She should have been more detailed in her question, but you should also have been more forthcoming with your answer. Kitchen Table play I would say you didn't break any rules, but that won't stop you from sleeping on the couch tonight. In the future I would be more detailed in asking and answering questions. If this was FNM I would explain when mana leaves the pool and passing priority of stuff.
The devil's advocate in me loves hearing and reading about these scenarios so I feel compelled to share my personal insights, opinions, and interpretation...
While it's courteous and good sportsmanship to do so voluntarily, it technically isn't and shouldn't be your responsibility to remind opponents (at fnm and other levels of sanctioned play) when mana leaves pools as timing restrictions are among the earliest and arguably more important rules one learns (or is supposed to learn) when they first start playing.
For reference this is included in the rules about mana itself:
106.4. When an effect produces mana, that mana goes into a player’s mana pool. From there, it can be used to pay costs immediately, or it can stay in the player’s mana pool. Each player’s mana pool empties at the end of each step and phase.
...and is repeated again for thoroughness in the rules pertaining to turn structure:
500.4. When a step or phase ends, any unused mana left in a player’s mana pool empties. This turn-based action doesn’t use the stack.
Suffice it to say that players should know this or know to ask/confirm if and when you're advancing to the next phase/step during a turn after any player has floated mana and there's a lingering sense of ambiguity since mana floating usually implies the controller has one or more plays pending prior to the phase/step advancement.
500.2. A phase or step in which players receive priority ends when the stack is empty and all players pass in succession. Simply having the stack become empty doesn’t cause such a phase or step to end; all players have to pass in succession with the stack empty. Because of this, each player gets a chance to add new things to the stack before that phase or step ends.
Some could argue that the very nature of her question indicated that she was passing priority in her desire to have your mana "go away" after which you failed to acknowledge that you were doing the same by answering her question with intentional vaguery (essentially a tactical delay of game which works to your advantage).
However (this is directed at the OP), this was at the kitchen table with family where your poor communication earns a symbolic poor sportsmanship warning in my opinion.
Furthermore, in your wife's defense, if she (or both of you) started playing way back when mana burn was still a thing (which I assume was the case based on you having a library to play with casually), only play at the kitchen table (or stopped playing for a long time while the rules for mana changes), it's reasonable to assume that she may not fully understand the adjustment (seeing as how she wasn't oblivious to the emptying of mana pools), which is a husband's responsibility to explain and clarify which you failed to do. Does it make you a malicious or cheating player looking to exploit an opponent's ignorance? Not technically though I suppose that's a matter of perspective albeit no more up for debate than whether or not you're a nice husband.
All that aside, you were exploiting by your own admission. You knew she had nothing but land in play and presumably couldn't/wouldn't attack, then you were asked asked about your mana leaving the pool (indicating or at the very least suggesting her intent to pass priority and advance to the next phase), therefore you should reasonably assume it was her intent to cast that *** during her second main phase after short-cutting to skip over the combat phase at which point you no longer had that mana floating.
For more information regarding tournament setting, the Rules on Player Communication (Section 4.1) can be found here.
More specifically, here's the relevant paragraph is about "free information":
Free information is information to which all players are entitled access without contamination or omissions made by their opponents. If a player is ever unable or unwilling to provide free information to an opponent that has requested it, he or she should call a judge and explain the situation. Free information includes:
Details of current game actions and past game actions that still affect the game state.
The name of any visible object
The type of any counter in a public zone.
The physical status (tapped/flipped/unattached/phased) and current zone of any object.
Player life totals, poison counter totals, and the game score of the current match.
The current step and/or phase and which player(s) are active.
Bottom line: I hope your couch is comfortable