See what you all need is an Irish system Public Health Nurse.
Last time we went it was the day before DD1's third birthday, so the doctor on duty gave her five euro as a present, and then remembered to send us the words and music for a song she was singing that made DD1 laugh.
The hands-down funniest moment today was when DS1, after days of trying, got DD2 to shout 'bum'. The poor child was quite frightened by the uproar that followed and couldn't be persuaded to repeat the feat.
Poppy the family stuff sounds so painful, and I've so been there with that thing of having a child attacked by a child who has good reasons for being upset or aggressive.
Could you plan something quite structured for them to do together next time? I freeze balls of cookie dough and bring them out when the tougher nuts come to call. They all love the activity and you can supervise quite closely without making a fuss about it.
Liberal use of the phrase 'In this house, we don't ...' is handy too. Your poor DD1
Sorry to hear things are still so tricky for your DSIL and BIL.
On a brighter note, we now have doors upstairs in our house. Cool, huh?