Husband in love with my sister

I'm pregnant with our third child. We've been together almost 12 years, married for almost 6. Due to covid, my college aged sister came to stay with us for two months to finish out her college semester online off campus and to spend time with my kids. Today, I found out my husband wrote her a love letter and made a handmade wooden box for her, complete with an embossed inscription for her birthday. I've had a weird gut feeling about a variety of little things over the two months but nothing I could put my finger on specifically or put into words. Two weeks ago, I found them talking late into the night (5 am) when I woke up to go to work and especially since it was like the third or fourth time, I told them it made me feel weird and inappropriate. They teamed up at the time to make me feel like my feelings were hormonal, crazy and paranoid. I confronted husband and he denied being in love ..but that he certainly has feelings for her.
They swear nothing physical happened and I believe this aspect of it. Sister says she does not reciprocate his feelings and that he first told her he had feelings in March but she didn't tell me this at all. Husband said he told her this to try to get it out of his system and get over it. She said she felt like because he said he was going to bottle it up that she felt it wasn't a problem anymore. I'm giving her the benefit of the doubt and going with "she's just that naive" but my gut feeling is that she borderline encouraged it (wore crop tops all the time around the house and really tight leggings and etc.) and didn't actively discourage it because she probably liked the attention.
Is this marriage even salvageable and worth going to counseling for?
TL;DR: husband is in love with my sister. Is this marriage even salvageable?


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