Geez...you've got some interesting dynamics / prior history going on and you just got there! Are you re-confirming your boundaries using a recent survey of your lot? If there are discrepant bits here and there keep us apprised as there are a lot of nuances and best approaches with property lines. Also, I wouldn't start marking off property lines with signs, lines, or other because it doesn't do much to elicit good feelings towards one another.