i think you need to tread very carefully and not just jump in the middle of things, such as taking this directly to your sister's ex. yes it is inexcusable that she "forgot" to pick up her kids, but i'm not sure that incident alone is enough to call in the SWAT team.
children of addicts/alcoholics/abusers most often DO defend and protect their parents.....their innate survival skills TELL them to do so....they also do not want to rock the boat and make things any WORSE. knowing also that their grandfather is a hot head and likely to bluster and yell, he is certainly NOT who they are going to reveal anything to. they are young and scared.
so let them know they are loved. that is all. give them warm hugs. tell them how magnificent they are, what you admire most about each of them, what makes them unique. don't do that all in one sitting, but make THAT your goal - that they get a good loving message about their awesomeness from at least one person, WITHOUT JUDGEMENT and without trying to manipulate them into to tattling on their mother.
don't make this about HER. her tardiness and flightiness is probably part of her overall MO....she doesn't strike me as the punctual type who honors commitments and gives a goddamn about anyone or anything.
if things go missing - money, jewelry, hockables - or if your mother's meds are decreasing at faster rate, then you have more grounds for suspicion. is your mom enjoying having your sister there, at least for a while?
if you TRULY feel the children are in DANGER, being neglected and/or abused, then that should be taken to the proper authorities.