Your situation and your WH's relationship with his sister are, unfortunately, eerily remind me of my ex and his sister.
My ex-SIL is arguably one of the most malicious and toxic people I've ever met. I could write a novel about her. My ex had a very co-dependent--borderline emotionally incestuous-- relationship with her. She was always jealous that I "took her brother away."
I always tried to keep peace with her but the final straw was when she invited herself on a cruise that my husband and I planned to celebrate my master's degree. I said no; this was a romantic getaway to celebrate my achievement. She threw a fit, my husband tried to get me to capitulate, but I didn't back down.
Shortly thereafter, she got in touch via Facebook with my ex's last serious girlfriend, who was still single. Ex-SIL then gave the ex my then-husband's phone number and voila... within a week, ex-gf was OW#2. Three months later, I moved out.
The standard advice on SI is that anyone who was a party to the affair needs to be cut off completely. The fact that you've realized that's unrealistic in this case and haven't demanded he cut contact with his sister is extremely generous and understanding of you, even if he doesn't realize it.
The very least that he should do is be transparent about the frequency and the content of his communications. He shouldn't be hiding from you when you calls and you shouldn't have to speculate about what they're texting.
Unfortunately, I really don't have any advice about what you could do to enforce these terms because I suspect that, even if he pretends to comply, he's just going to do whatever he wants to do behind your back.
Ultimately, it comes down to whether you can tolerate being in a relationship with someone who keeps secrets from you and values his relationship with someone who tried to destroy your marriage over his relationship with you.
[This message edited by BluerThanBlue at 6:40 PM, Tuesday, May 7th]
BW, 40s
Divorced WH in 2015; now happily remarried
I edit my comments a lot for spelling, grammar, typos, etc.