if the 25 year old received the same (lack of) care and attention from her mother that her 3 year old sister seems to be getting whilst she was growning up, no wonder she is immature and unreasonable!
although i think it is and the way this girl handled the incident, i would maybe let the dust settle a bit? if you speak to them now things could escalate further and you will still have to live near them..
they sound a rather shocking family