Jennifer probably wouldn't be thrilled to find out that her boyfriend has been macking on his own mother. She seems to really love him and all, but every girl has limitations of tolerance. And most would agree that "parking" with family members isn't exactly kosher.
But, because she's the only significant character from 1985 that we don't see in 1955 (she wouldn't be born for over a decade) she's able to remain blissfully ignorant about everything that goes down. The two end up getting married, so we have to wonder how honest Marty is with her about his distant past… but they seem to be making it work, so who are we to judge?
By the way, anybody else think she plays it a tad too cool when she's asked to hop into a time machine so she can visit her future kids? For someone who's hearing about this for the first time, we think an on-the-spot fainting or freak-out moment would have been more appropriate.