Consider that if the genders were reversed, 99% of Reddit would be saying to turn the cousin in and not do it. But since you're a guy they're saying smash it, finish on her face, etc. Just saying, consider the double standard (and next time, reverse the genders to get the Reddit hive mind's true thoughts).
My advice: If it feels wrong, don't do it.
What's a cousin by marriage anyway? Is that like, when your aunt or uncle marries someone who already has a kid, that kid is your cousin by marriage? I've got a few of those and have never thought of them as less than family. But that's just me. And the problem with incest isn't, as kids my age (1970s) were taught, that if the girl gets pregnant, the kids will be deformed. There's protection, there are abortions, and not all unprotected sex results in pregnancy. STDs are a bigger threat, especially if you're not the only guy this cousin is fooling around with (sounds like you're not). No, the problem is that while it might be fun having a live-in lover, when it's time to break it off, you can't get away from the person. Your home's no longer a haven.
That said, regardless of what you do, you're always going to have sexual history with this cousin. It's not gonna matter, "Oh but I never finished inside her vagina." That's a mighty fine technicality when she's made you finish with her hand (and maybe you've done the same for her?). At that point, it really doesn't matter, so if you're okay with that... outside of the fact that she's pressuring you into something you're not comfortable with, if that wasn't an issue, I'd just say that ship has already sailed. Oh, I just re-read and saw that mutual masturbation already happened. Yeah, you've both came in each other's exclusive company, you've pretty much already had sex with her, just in a different way.
Oh, and if you want her pressure gone for good, hook her up with a friend. Better the friend the better. When/if they break up and she comes back around, cite the bro code at her. Friends' exes are off limits. Bro code is practically ironclad.