Senior dog and puppy

We got a puppy about a month ago when we had 2 senior dogs (both 10) one of them was not a fan of the puppy (oldest) and the other was mostly indifferent. The eldest dog passed last week and now the other is not attacking, but lunging at and barking at the puppy. Is this my life now of having to manage between the two? We love the puppy, BUT my elderly dog comes first and if this is stressing her out that bad I don't know if keeping the puppy is a great idea. I don't want her to have to live her last few years of life like this. I do separate them when necessary, crate the puppy if he's doing too much or send her to the bedroom to rest. They play together at the park fine and go on walks together fine also. Could this maybe just be a phase of her grieving her sister? She will go after him even when he's across the room and not bothering her at all. Her and her sister would occasionally fight, but until the older one developed dementia it was rare. She's never been aggressive towards other dogs. I worked in vet med, rescue, and pet sit when she was younger and recently started again after having kids so she's been well socialized. She is on joint supplements and pain meds. All bloodwork was good. Kidney function was just checked and it was also within normal limits.

I don't think it's fair to either one to have to spend time being "cycled" through the house. Other people can and have, but I don't want her spending her last years that she should be relaxing to be full of stress. I also have young children and have had a reactive dog before so I know management can and will fail. I don't fear for my children at all and she's not attacking the puppy per say. I would almost say correcting, but she does it when there's nothing to correct just if he's walking in the same room. I partially think that she wants to protect the kids because he jumps on them and is normal puppy nippy.