Hi Soldadaromana
BPD is a challenging disorder and I understand your concerns for your sister and her children. You mention his medications, has he perhaps been officially diagnosed with BPD and/or another disorder?
One of the three children in particular is the scapegoat and is treated badly by the father, which breaks my heart.
... .
... .he has threatened to kill himself and he has even tried to strangle her on one occasion at least. She makes an excuse for this by saying that he was not taking his medications during that time
It is very sad that he treats that one child this way. Often BPD parents engage in the behavior known as 'splitting' and they often designate children either 'all-good' or 'all-bad'.
It is very concerning that he has tried to strangle your sister. What did your sister do when that happened? Did she alert any authorities or did she totally excuse it because he 'wasn't taking his medications'? We have some material here that I think both you and your sister might benefit from:
Safety FirstHas your sister perhaps ever called a domestic violence hotline?
Take care and welcome to bpdfamily