How do you do the drawing part? I'm guessing you create new sprites at the mouse/touch position? Simply add new points to the spline path at the same time. Just make sure not to add two points with the same coordinates, Spline doesn't like it.. See this post:
https://www.construct.net/forum/construct-2/how-do-i-18/hi-i-have-a-problem-with-rex-s-136098
Many many thanks DOP, it helped me fix the issue. God bless you.