You'll find uneven studding in lots of old houses. When using plaster it wasn't necessary to have a perfectly flat structure underneath so nobody bothered to make it that way. Instead of shimming or sistering with 2X studs, it the gaps are not more than about 1/2" at the worst places you can 'sister' with 1X strips then screw the drywall into the studs, not the added wood, whose sold purpose is to flatten the wall. Cheaper and adds less weight than 2X sistering, this works well since the drywall is stiff enough to not bend where screwed on. I usually do this with 1X6 ripped in half which provides adequate strength against splitting but won't hit anything on the other side of the stud. Either piece it or notch for pipes and wires. Do similarly at the top and bottom plates.
Unless I have to demo a whole plaster wall for repairs underneath I prefer to "overlay" it with 3/8" sheetrock instead, letting the existing plaster form the flat surface needed. Glue and screw the drywall on then finish as usual. Door and window moldings will need to be furred out to match the thicker wall but this is usually a lot easier, faster, and cheaper to do and it works well enough to be considered a permanent fix.