Advice for best route

I adopted a puppy, she is 6 months old. I felt pressured into it (a story for another time) but love animals and dogs so decided I could figure it out. It’s been a month. There has been typical puppy oopsies, like tearing up things and potty accidents. I have tried to be strict when training her but there’s no difference from day one to now. In fact, I feel like she knows less and listens less now. I’ve tried to tell myself that training and learning can be figured out and takes time.

But now my main concern is that she has been starting fights with her older sister. This dog is 3 and I have had her for a year and half. 90% of the time they get along but the last few days they have been getting in horrible fights. Neither one backs down and it’s truly like they want to kill each other. The older dog now doesn’t want to eat. I’ve been able to get her to eat treats and bones over the last few days but now she doesn’t even wanna do that because of the puppy. I’m worried for both of their safety, even when I’m around, as these fights have happened with me around. I’m also worried about the well-being of my first dog. I can tell she’s been more depressed and angry since getting the puppy but has tolerated her but I fear it’s getting worse.

Giving the puppy back breaks my heart, as I never thought I would be that person. But i truly am worried about my first dog and obviously both of their safety. Giving the puppy back is the last resort, but i need reassurance on if it gets better for my older dog or if the fights will continue to escalate and get worse. I never wanna make an emotional decision but need to do what is best for both of them and idk what that is right now.