The oldest son usually becomes the head of a family in Japan.
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means inherit and the verb vocab version is to inherit.
You not only inherit one fortune, but two! You're lucky both sides of your family "accidentally" died at the same time. TWO (つ) fortunes! TWO! HAHAHAHA. Also, you can think of it like the つ of 次, because you are the "next" successor when you inherit something.