My bathroom sink just started backing g up into my bathtub have been struggling with the toilet doing the same thing for months how can I fix this and what could be causing it?

Partial blockage in main drain line. Pull toilet and snake the line.

Agreed or snake down the vent stack on the roof if it is safe to do so.

Sewage shows in the tub first because tub has lowest drain... As said above, snake the line

Milo