Is My Husband's Half-Sister's Best Friend Trying To Steal Him? - I Need Advice

I believe my sister-in-law's best friend is trying to pursue a romantic relationship with my husband (28M) and she's actively encouraging it. Am I being paranoid, or is there some truth to this? My sister-in-law (23F) is actually my husband's half-sister, a product of their mother's infidelity. Unfortunately, my father-in-law has disowned my sister-in-law but continues to maintain a relationship with my husband. This has made my sister-in-law extremely dependent on my husband, and he, in turn, is fiercely protective of her, never tolerating any negativity towards her.

Last year, my husband and I bought a house with its own indoor swimming pool. When my sister-in-law saw it, she was thrilled and asked if she could occasionally use it with her friend Chloe. We agreed, as long as she gave us notice before coming over. Consequently, they have used the pool multiple times, although rarely when my husband is at home.

But now, with the lockdown in place, my husband has been working from home. This situation has given my sister-in-law the opportunity to ask me if she and Chloe could use the pool more frequently, even when my husband is home.

This has left me feeling uneasy and suspicious. I can't help but wonder if there's something deeper going on between my husband and my sister-in-law's friend. Am I overreacting, or is there a genuine cause for concern? I need some advice and perspective on this delicate situation.