As a parent of a preschool age kid, I do think that's too young to have a personal screen device, sorry. If one of my siblings or my parents did this without checking with me I'd probably ask them to return the devices and explain that we'd like to hold off for a few years. I wouldn't hang onto the device with the expectation of using it later (and not using it at all now), as there may well be a newer/better model out in 2-3 years time. Personally, I would insist on giving it back to make the point that it's not okay to give my kids gifts that are basically going to be a ton of work for the parents without clearing it first. I think if OP had posted about his sister buying the kids a pair of big screen TVs for their bedrooms the reaction on here would be much harsher, but without effort on the part of the parents, it seems like this is the same kind of gift. I don't think you're going to see too many people saying "oh, I would be so much better off as an adult if I had my own television as a kid!"
If asking your sister to return the item would cause friction you don't want to deal with, I'd say you need to commit to sitting down and spending a few hours figuring out the parental controls (which you are going to have to do at some point in the future anyway unless you want your kids to have zero screen access ever) and set it to something you're happy with (e.g., use for checking out library books only). Then also control access to the screens (have them living on a high shelf or something) and your kids have to ask you for them, so you have an idea of how much they are being used and can control use. I would also limit where they can use the screens (no disappearing off into their bedroom for hours, for example).
Having seen some young kids who simply cannot be ripped away from their personal devices without causing a huge shitshow, I think it's perfectly reasonable to want to keep a screen-free home or at least limit access somewhat by not having personal devices and monitoring/limiting access to shared devices. I want my kid to have a good idea of how to explore and entertain himself WITHOUT a screen before I introduce one into the mix. For young kids especially, there are a lot of motor skills that you learn and practice through things like physically handling books (or Legos, blocks, puzzle pieces, etc) that you aren't going to pick up by tapping a screen.