First of all, the SDA publicly claim to uphold the Trinity. Trinitarian language is used all over their writings. The problem is that they redefine those same words, and thus invent a three-god theology as a result. The information I have shown you are from SDA theologians, as well as the writings of Ellen White and the rewritten scripture of their "Clear Word" Bible.
I would challenge your DiL (I don't know what that stands for) to define the Trinity, and see if her definition fits with the Biblical teaching.
Nice talking to you.