I've got a somewhat strained relationship with my DSis. Although we weren't close growing up, we became so in our 20s, but since we both got married and had DC things have been strained (although she is younger than me, her DC are older as I had infertility problems). There is no one reason for our strained relationship but a whole lot of small ones combined with some personality clash. Overall though I feel the main problems are that she is jealous of me (especially my career - unfortunately we are in the same line of work but I am more senior due to different decisions and less time out). Since I had my DD it has been impossible to find the time to do the kind of 'relationship maintenance' I used to with DSis.
I think she probably had PND although she wouldn't discuss it or seek help. I tried very, very hard to be helpful and supportive during the 5 or 6 years that she was having problems and got shouted at a lot and told 'you couldn't possibly understand', which I didn't realise at the time could be a symptom of depression and in any case I found hurtful because of the infertility - we had to have about 5 years of treatment to have our DC. Her reaction when I told her we were ttc was 'but you are my role model of a successful career woman who doesn't want children' 
DSis seems better now, has a job and some hobbies, but although we meet up every few months as our DC get on very well, every meeting seems to end (or start!) with us upsetting the other in some way and words being said. I am also finding it hard to move on from some of the things she did and said when she was having problems. She never refers to this period - it is almost as though she doesn't remember how bad it got at times.
To be honest, if she was a friend rather than my DSis and if it wasn't for the DC getting on, I would have given up by now. I work very long hours in term-time and just find the whole thing painful.
I don't generally have problems getting on with people but I am sure there are times when I haven't helped things with her by for example giving careers advice (generally only when asked, but I also realise now that probably I should have just sympathised and not tried to solve problems).
The problem is that our parents are getting on and it is beginning to look like they need more support. I live nearer to them but DSis has more time available. I feel like it would really help if DSis and I could form a united front. I am despairing at the thought of trying to work together with her to support our parents (DH is civil to her but finds her annoying and sometime rude too...to be totally honest we are both a bit scared of her when she is really cross). DPs attitude can be summed up as 'DSis is who she is, deal with it'.
I would like to know any tips anyone has of how to improve or mend a sibling relationship like this and any dos or don'ts regarding trying to work with a sibling to support elderly parents. I think my DSis would like our relationship to be better.
Sorry for length.