Dating My Best Friends’ Brother

I've found myself in a delicate situation. I've developed feelings for my best friend's brother and we have been dating for about 2 years, but there are a few hurdles to overcome. She's the older sibling, a bit controlling, and tends to be quite nosy. Whenever I try to casually mention her brother, she quickly changes the subject or makes excuses.

I know there's some underlying tension between them, perhaps stemming from their mother's passing. He might fear that our relationship could jeopardize their bond. As for my bestie, I'm worried she'll overreact, thinking I'll spill her secrets. But I'm confident in my ability to keep things private.

For example, my bestie shared some stories to me that I thought were true about her life growing up but her brother revealed those were lies. It’s crazy to me that she would lie about simple things, but It's a complicated web of emotions and suspicions.

What would you do in my shoes? When would you reveal your feelings to your best friend? Given our age, I don't think it's a major issue, but is it really that simple?