Well, you're an adult, and you have an adult problem. The thing that you should do is to get your dad alone, and tell him that Emily has started texting you, and that you'd prefer not to be a part of his "other" life, as it makes you uncomfortable.

I suspect that he'll accept that info from you and handle things from his end, and you'll no longer get texts from Emily. If not, simply block her.

However, there needs to come a time when you disclose to your bf the dynamic that exists in your family, and the reasons it exists. I know you're hesitant because of the dynamics of his family, but his family is an ideal to strive for.

As for your father, it's obvious that he was unwilling to live in a "sterile" relationship, and sought love outside the marriage, and no doubt your mom is fed up with the relationship as well. It would probably have been better for all involved had a divorce taken place regardless of a sibling that's on the spectrum. Had that occurred, your mom and dad would now at least be amicable.

I wish you well.