First off ... the OP said,
"Last night however, my DH says that he talked to his sister (lives in Orlando) and she said that her DIL's baby shower is going to be next weekend. She says that they wanted to have it while we were in Orlando!" DH called his sister, so it's safe to say they at least like each other. DH apparently did not automatically say, "Oh man ... I'm sorry, but we won't be able to be there", so it didn't instantly strike him as "OMG! Taking away my family time!" And who knows? Maybe DH had been told a long time ago -- like when he first told his sis they were going to Orlando maybe? -- and he FORGOT to tell his wife until his sister reminded him last night? It might actually not be the sisters fault!
The sister knew her brother and his family were going to WDW far enough in advance to schedule a shower for the weekend they'd be there. Could be that she'd mentioned it to her dear brother a month ago and he (being a man and not pre-disposed to going all gaga over showers) forgot to mention it.
Secondly ... OP keeps saying that it has nothing to do with losing park time or Disney time, it has to do with losing "family time". Well ... these people are her family too (extended though they may be), so if she has actually SAID this to her DH, he may be confused thinking, "I thought my sister was family too", hence him not immediately saying, "Yes -- you're right! Immediate family only!"
"She shouldn't be expected to drop her plans and go running to them." Where in this whole thing does it say that the Sis expects anyone to "drop her plans and go running to them"? They were invited to a shower, for goodness sake, not demanded to provide an audience. It's entirely possible that the Sis wasn't thrilled about it either, but when her brother (the OP's DH) called and said, "we're going to be in Florida," she changed the shower because she felt she was expected to include her brother and his wife.
"They screwed up and now they are left with the result of their poor planning." How have they "screwed up"? They changed a date so that they could include more family. Sounds like they might actually
like the OP and want her to be included.