Let me start by defining some terminology. Many questions in fiction are direct questions:
“What time are we leaving for the carnival?” Stewart asked.
When a direct question is reported, it becomes an indirect question:
Stewart asked what time we’re leaving for the carnival.
Indirect questions don’t use a question mark or quotation marks.
Your example starts out as an indirect question, but retains some of the syntax of a direct question. The direct question Lisa asks is likely something like this:
“Why didn’t my mom get the letter yet?”
Your example retains the syntax of this direct question, but simply replaces “my” with “her.” This might be why you feel compelled to keep that question mark; it sounds like a direct question.
An indirect question—one that’s reported—would read a bit differently:
Lisa asked why her mom didn’t get the letter yet.
If you intend that line to be reported, adjust the language accordingly and lose the question mark. Then, get back to the important business of that letter. Where is it?