Serious problems with partner's family. Advice/opinions welcome.

So this is going to be a long one but please stick with me!! My partner's dad and sister have flat out accused me of deliberately getting myself pregnant to trap my partner.

*BACKSTORY*

My partner's dad has recently come back in to the family home after admitting to a 2 year affair with another woman and packing his bags and leaving his family high and dry. During the time when he left the family, I was an integral part of the support for my partner's mum, travelling 2 hours by bus to make sure she was okay, keeping her company and doing bits of shopping for her as well as being an emotional rock. My partner's sister wasn't around at all to support her mum, and only came round a handful of times (begrudgingly) and at the time I felt like I was doing what she, as her daughter should have been doing (not a complaint, simply an observation).

Fast forward to now and my partner and I have found ourselves unexpectedly pregnant - I was on the pill.

We held off a few weeks to tell his family as they had been through a lot this year. My partner told his parents first. His mum was very supportive and accepting, his dad was supportive but wasn't happy for us. (I am 20, my partner is 27. We both work but haven't got around to moving out together, that was the plan for 2018. We have been together 2 years this year). His sister was next to find out, the following day. She spent the rest of the week giving my partner the cold shoulder, not speaking to him, avoiding him. The weekend rolls around and I am told that I am not welcome to spend the weekend with my partner and his family as I usually would as his dad and sister "don't want to see me". Fine. Okay. Whatever. So my partner came to mine instead.

The next weekend comes around and I spend it at my partner's house with his family. This is the first time I've seen them since they found out I was pregnant. His dad stands up in the living room and basically says that I've done this on purpose to try and trap my partner. I was absolutely gobsmacked. As it transpired, my partner's sister has put this idea in to his head by telling him that a mutual friend of ours has told her 6 months ago that I've been going round telling people at work that I was planning to get myself pregnant deliberately. Which I would like to make absolutely clear I have never, ever, ever said in my life.

*2ND BACKSTORY*

The person who has said that I've been saying this has been causing me trouble at work and between myself and my partner. He tried to convince my partner I was going to beat him up and also tried to convince people I was capable of murdering someone, things that, of course, have been proven to be outright fabrications. 6 months ago, my partner was also told what had been said who knew then and there that there was no substance to it. He never asked me about it - this is the first I've heard of it, he never insisted on wearing a condom or abstaining from sex AND he's said that he absolutely knows it isn't true.

My partner's sister, after avoiding me all weekend finally comes to speak to me, reiterating what she has been told by the mutual friend and tells me herself that she thinks I've done this on purpose. I tried to explain my side of the story and that I didn't think accusations were fair. She stormed out and that's the last I've spoken with her.

I feel totally and utterly betrayed. I don't know how my partner's dad can stand and accuse me of deception and lying after what he's done to the family and as for the sister... I did so much for her in terms of support when her relationship broke down and I looked at her as my own sister. We think she is jealous as my partner is her twin and we have what she wanted with her (now ex) when we didn't want it now.

Thankfully my partner and his mum both know the truth and are so so so supportive but I've said that until I get an apology and the other two accept the truth then I do not want to be part of the family. We all know that as soon as the baby is born they will want to be all over it.

If you've read to the end then thanks for sticking with it. Advice, opinions, comments will all be well received. Anyone had anything similar happen to them? Or any other dramas with your other half's family?