You have time until the snow comes so get this under control now. Leave early and walk her to the bus stop. Every time she pitches a fit leave her to do so (you stand with the DC kids in the stroller, ignore the tantrum) when she's done then continue walking to the bus. If you are rushed/she feels rushed, this won't help matters.

Or- Can your partner/MIL/neighbour leave for work a little later the next 2 wks to walk her to the bus? Do you have someone that can watch the DC kids for a brief time while you take her just until the routine is in place?