Any advice or experience?
My mother has always favoured my younger sister. Every so often she will mention that my sister is very similar to her, that I'm more like my father (she's seeing what she wants to see imo).
My sister is quite lazy, she does a job that is far beneath what she could do if she made the effort and applied herself. I'm the opposite, I've always worked very hard and put mysf under a lot of stress, I'm sure part of this was due to 'never feeling good enough' but dm doesn't value this. She always pays for dsis and mollycoddles her. She looks after dsis baby one morning each week, she's never offered to look after any of my children, I do live an hour away but I'm a single parent whereas dsis has a husband. I don't expect her to look after my children, it just feels she treats is very differently.
Another example would be that I paid for us to go on a weekend away for her birthday. Dsis didn't bring any supplies (snacks, drinks etc) but when we went to the supermarket dm asked me to buy various things, whilst dsis slunk round silently, avoiding ever having to pay. Then when we went for lunch dsis didn't even bring her wallet! I've argued with dsis about freeloading before but she's totally protected by dm.
In short it may sound pathetic as a grown adult but I'm actually really jealous of dsis getting all the attention and favouritism and not being able to do any wrong, whilst I try really hard to do everything 'right' but get ignored completely. Dm hated her older sister and has likened me to her in the past 
so maybe that's why she feels so close to my younger sister.
Fwiw my friends have also seen the favouritism and have said dm mollcoddles dsis and that dsis is very lazy and moody and expects to get her own way all the time.