How to deal with a Taurus SIL

Ok, so I'm a Scorp and I've been dating my Taurus for about a year now, and some issues have arisen with his sister (who I will refer to as the sister-in-law, loosely speaking, of course... and she is also a Taurus). I've been introduced to his family, and we were getting along wonderfully (or so I thought). Then I got a call from the SIL who bashed my character for being too shy; who finds it disrespectul that I've worn pyjamas around the family (his mom got me a pair of PJs *once* because I wasn't feeling well, my stomach distended to the point that I looked like I was preggers, and so she wanted to make me comfortable); and she insults my bf behind his back to me. She takes it upon herself to tell me what's right for her parents (even though her parents are more than capable of communicating with me effectively), and she hates the fact that I don't come from a family like hers, saying that if my bf stays with me, he'll never be as successful as her.

So that being said, I've laid down some groundrules (which I don't usually do because I'm not overly assertive) - 1. no speaking on her parents' behalf because I hate broken telephone. 2. no bashing my partner to me - any issues they have, they should sort out on their own. 3. don't tell me that my behaviour is "disrespectful" as I'm still learning the code of behaviour around his family.

I've told her that I would like for her and I to have an amicable relationship, but both our rights and boundaries need to be respected.

Am I dealing with this properly, or is there a more effective way to deal with the Taurus SIL? Do I need to be equally as stubborn in order for us to both respect each other? Or should laying down the groundrules suffice?

And for those who will argue that my partner needs to step up, he's tried. He's defended me, but she constantly berates him and puts him down, which angers him to the point that he doesn't want to hear her anymore. He's almost developed a phobia of dealing with her because she's been treating him this way for years - saying the decisions he makes are not of moral character or of good judgment (he's intelligent, he's a successful professional in a completely selfless profession, and yet she still takes objection with everything he says or does). I've told him that I can defend myself just fine, but he'll need to step in at some point and resolve his own issues.

Is there anything else that can be done or should be done?