Although other projects have been rumored for Disney+, including this one and a Scarlet Witch project, to me the fact that Disney is keeping mum about what is in early development could mean no hard decisions about what projects are moving forward have been made yet.
To be clear, currently Disney and its streaming service are not confirming a Lady Sif streaming show is in the works.
Currently, Disney has only officially confirmed there is a Loki-oriented streaming project that is coming together.