As the parent of a speech delayed toddler I can hand on heart say that whenever someone "gently lets me know they think my son has a speech delay" it makes me want to throat punch them.
I'm sure your sister is aware.
When people bring up the obvious, it completely detracts from any other enjoyment you have with your child.
Just because some children don't speak, it doesn't mean they don't understand.
Just because some children don't speak, it doesn't make them any less worthy of love, attention, nice things and good experiences.
Just because some children don't speak when you expect them to, it doesn't mean they won't ever speak at all.
Just because some children don't speak coherently enough to be understood, it doesn't mean they aren't trying to speak or communicate.
Just because some children are still learning to speak, if doesn't mean they can't convey their wants and needs through body language, sign, crying, laughing, pointing, pulling and pushing.
If you really want to support your sister and believe her child is showing signs of delay, then don't probe your sister. Participate in more activities with the toddler that you know are good for cognitive function like singing, signing, taking them out to experience more of the world, read lots of books and talk about what you can see in the pictures.
I can promise you that your sister would value that kind of bond being nurtured than being told her child has a delay.