What's Happening?

I am a little confused to why Saturninus announced Lavinia as his wife. Did he not know that his brother, Bassianus loved her? Then to my surprise, he declared Queen Tamora as his bride, but isn't she the Queen of Goths? Wouldn't that make her an enemy against the Romans? I would assume so because Titus is a Roman and the general AGAINST the Goths. And wasn't she some sort of hostage five minutes prior to that? This play is going to get twisted, and I say this because Tamora only accepted Saturninus's proposal so that she can get close to Titus and play out revenge for making "a queen kneel in the streets and beg for grace in vain" (I, i, 457-458).