My mother could have written this post, minus the pandemic. My sister is six years older than me. Basically the phases were (as Mom tells us): older sis was infatuated with me, treated me as her doll, was inseparable. Then older sis decided she was jealous of all the attention the baby of the family got and was resentful. And lastly (around middle school), pretty much indifferent and had her own affairs/teen dramas to deal with.
I have nothing but happy memories of my childhood even though we weren't terribly close as siblings then.
As adults, we are much closer than we were as kids. We love our shared history and enjoy each others' company.
I think what you are going through would be happening even in a normal life situation, outside of all this stay at home chaos.