It looks like Tver is a Rival of Muscovy. Paradox's dismal failure at attempting to make Succession wars more interesting has once again caused issues. Not only is it limited to letting massive powers force unions on countries they have no dynastic ties with, but now this.
An intelligently designed system, baring the addition of a proper
Character System would allow for the following situations;
1: Succession war between two countries who both have dynastic ties with the target. Behaves like old Succession Wars.
2: Succession War between a country with dynastic ties to the target and country A's Rival. Country A attempts to force the Union, Country B attempts to dissolve it.
3: Succession War between County A and it's Target. If the Potential Union Leader is a Rival of the Union Junior, then instead of immediately forming a Union it just gives The Potential Leader a "Force Union" CB.
Seriously Paradox this isn't hard.