It sounds to me as if you are in a position I was in a few years ago. You are facing your dear dad having palliative care and it is worrying for you which will make you extra aware of what's going on. Sometimes, under normal circumstances, these events don't challenge us but when they are serious we have a heightened state of awareness. For me, I would concentrate on my dad and the precious time you spend with him while you can. Your SIL knows what it going on, she is older than you as you say, let her find her own level, be it whatever it is. Sometimes, when times are tough we see the real person in others and sometimes we see how kind they are. If she is over quite an age, perhaps put it down to lack of connecting the dots anymore. I have a friend of 85. Last year we had a great time and this year, I had not seen her for six months, she had become very negative and moany. Focus on your self care and the care of your dad at this point in time. You will look back with fondness at precious memories.