opinions needed please

I have always had difficult in understanding my partners relationship with her brother, my partner. It has alwayhs been strange,like she is jealous, or she needs to show me thas she is the closest person to him... Tom my darling and l have been together for 10 year, and now his sister seems to be encroaching on our relationship, wanting to be the third party, actually wanting to be the most important person in the relationship. EEEk im begining to hate her!   I have so many things running through my mind with what she is doing in our relationship. l dont know where to start) My question runs rather deep, on lots of levels that l have not explained,His sister is in an unhappy marriage and plays the perfect marter, the all giving woman, ready to give all for her family, but she forgets the most important person originally in her marriage, her husband. He is a good man, and they have grown apart.I said to her that, my thoughts are, if it is broken try to fix it, if it is damaged beyond repair, leave the marriage, or at least in the first instance give it a try. Her response was lame, and all about her, how much she has given in the relationship, but dit not at all acnowledge anything her husband has done ( and he is a good man i know and see that, he is just boring and offers no excitement  She shows massive transferrance onto my partner her brother.  Its like what she cannot get from her husband, she wants from her brother , not sexually but in everyother way Its all very weird, and l just need someone to bounce my thoughts off She is so polite in public to me, and shows the world the why would i have a reason to doubt her.    As she justifies herself with giving my family and myself food, because we are not so well off as she is and her family, but makes it clear that this is food she is going to throw out, in the bin because no-one wants itShe is a wolf in sheep clothing.She plays the marter, always!!    So many things l could say now Like what was that all about on the weekend , enticing her brother over with stippers etc its totally fucked really