It depends.

If it's your own sister, then you learn to deal with her. If anything, adding a sister is fine as long as you have the resources. Removing a sister has legal proceedings upfront or after the fact, depending on your method.

Switching between romantic partners who happen to be sisters with each other can be risky! Lingering affection, jealousy, and inadequacy may all come into play here.

Personally, I dated one sister only to find that the younger one fancied me after well after we broke up. I found it difficult to separate the relationships and never pursued it. My brother-in-law dated my other sister-in-law first, and moved to his wife after a mutual understanding.

Finally, romantic entanglements with your own sister is discouraged. Not only is it illegal in many countries, having a child with your sister invites genetic disorders.