Thanks for all your replies. I've not heard of Makaton and I could look it up. It can be a difficult situation. I think there is some help through the school, but I have a fear one has to be quite proactive. I think in the meantime, I have to hang back, and just offer help when I'm asked for it, as I know my sister and our relationship, and I suspect sending links might be a bit too much.
The tricky thing is that I think it may be dawning that
DN will have SEN rather than just being a bit "delayed", and none of us in the family know how the best way to deal with it is - everyone has their theories about what worked with their child, but DN may well be different. I think we all, included my DSIS had expected DN to be "delayed", which is quite a nice word - nothing to worry about.
I think our parents have found it hard, as they had two children where, within reason, everything was easy (talking, walking, potty trained, etc), but those techniques probably won't work with DN.
The potty training is a big one. My DSIS knew that DN wasn't potty trained at 3.5 when he came to their family. However, at 5, he still isn't potty trained and doesn't seem to be interested in it, from what DSIS says. The same is true of forming sentences, rather than individual words: "carry", "cuddle", "again".
The sad thing about TV is when he came to my house once, he saw the TV wasn't on and pointed at it and said "broke?".
My DN was neglected for the first few years of his life, so he has had a really tough few years. My DSIS and her DH are showing them love, and there is a supportive network around. I think we come from the idea that "it takes a village to raise a child", but as a family, we have to step back and not impose our thoughts onto my DSIS and her DH.