Grandma thinks my daughter has a learning disability

So basically the title says it all. My mom thinks my second daughter is "slow" (as she says) but i know she's only a teeny bit behind where she should be. My oldest was way ahead of the charts at this point, so i do see the difference and i can see where someone might make that assumption as she's shy around strangers (she has met my mom a total of 4 times in her 2 1/2 years).

She sings a lot, listens and responds WHEN SHE WANTS TO. She's difficult to handle because she doesn't care about rules, punishments, etc and isn't focused enough to "have a chat" about why no means no yet.

Now, i do agree that she's not ahead of her curve at all. She's more or less just scraping by with her communication skills. But that doesn't make her an idiot!!! That makes her HER, and not her big sister! Instead of trying to diagnose what i know isn't a problem, i'd rather wait it out and continue to remind everyone to STOP with the baby talk that's likely the reason she's not communicating well.

Just had to rant.

Does anyone have any ideas on helping a strong-willed, rambunctious almost 3 year old learn to converse properly? Getting her to actually listen to 'no' instead of acting like it's a game when she doesn't want to? Tips on stopping her from getting distracted and running off to do something else at the splash pad are also welcomed 😉