No etymology is suggested. A Hebrew root such as snn or śnn would be ideal.
ṣll, “to lift up, exalt, raise, gather, cast up (into a heap)” or ṣlh, “to lift up, suspend (a balance), weigh.” See also the more likely post biblical Hebrew ṣnh (=Arabic ṣny), “to lift up, elevate” (JAT), though I have not been able to find this root.
If an Egyptian etymology is sought, the most likely candidate is the sniw (JG) a unit of currency which during the New Kingdom in Egypt was worth about 5 deben (Janssen, Commodity Prices from the Ramessid Period, 102-8). There are two problems with this candidate. The first is that attestation after the New Kingdom is wanting. The second is that it needs another n.
A possible, but unlikely, candidate for the origin of Senine is Egyptian snw, a kind of jar (JAT), though the unit of measure, volume, is not the same as the Book of Mormon unit of measure, mass.
Cf. Book of Mormon Senum, Seon