Thanks for sharing your concern here on PC. I don't know what has caused you to develop the reaction to your sister's bra strap you're experiencing. And, personally, I am loathe to speculate.

One thing I will say is that asking your sister to wear jackets or jumpers so you can't see her bra strap is really putting your problem onto her not dealing with what is actually your problem. (From what I've read, though, avoidance is not uncommon in people who struggle with OCD.) However the more appropriate approach, I would think, would be for you to address what you're experiencing rather than to expect your sister to solve the problem for you by changing what she wears.

Since you mentioned you have OCD here are links to 4 articles, from PC's archives, on the subject of Exposure and Response Prevention therapy (ERP) which is, as I understand it, a first-line treatment for these types of concerns. One thing to keep in mind with regard to ERP therapy, though, is that it is important to find a therapist who specializes in this type of therapy:

When to Seek Treatment for OCD

ERP Therapy: A Good Choice for Treating OCD

Exposure and Response Prevention Therapy for OCD

Anxiety and Exposure and Response Prevention (ERP) Therapy

Best wishes...

P.S. Family therapist Kati Morton has a video on her YouTube channel on the subject of ERP therapy. Here's a link:

YouTube