Enlarge the crack with a hand bottle opener, the pointy end, making the bottom sides of the crack wider than the front. A key of sorts to hold the new material in. As brought out in the article I listed.
Here is some history on plaster with the different backers: http://www.nps.gov/history/hps/tps/briefs/brief21.htm
The crack above to the left of the water damage suggests lath underneath due to the nature of the crack. When lath board under, the crack is different in design. The window trim dates it as mentioned to lath at that time. BUT, I may be wrong, either way, the fix is similar. If a deep area, the article mentions using a bonding agent for the fast, quick-set mud, but not absolutely necessary, just wet with water. After removing all
plaster. The final coat should be with regular drywall premixed mud.
Be safe, Gary
Clothes taking longer to dry?
Clean the dryer screen in HOT water if using fabric softener sheets.
They leave a residue that impedes air-flow, costing you money.
Clean the ducting in the last six months? 17,000 dryer fires annually!