Wow really? In ME 2 (Shadowbroker DLC), I found a bit strange that even if you'll admit you have something with Tali, Liara and Shepard still share intimacies while it doesn't affect the romance with Tali in anyway.
I think that is because Shepard is technically single in Mass Effect 2, no matter who was romanced in the first game. Death ends relationships. The Virmire Survivor never expects to see Shepard again and has likely moved on to some extent. (Kaidan IIRC says something about having gone on a couple dates) And then there is the whole Cerberus reveal on Horizon where Shepard basically gets told to get lost. Liara is a little different in that she was involved in the preliminary stage of the Lazarus Project (Shepard's recovery), but even then she couldn't have expected that it was guaranteed Shepard would return or would be the same person. Cerberus was attempting something that had never been done before. And when Shepard & Liara cross paths on Illium, she makes it clear that while she still has feelings for Shepard, she isn't interested in picking up where things left off at that time. She's reluctant to risk having to mourn him or her again.
That Shepard & Liara don't rekindle their relationship in that first reunion on Illium, and largely because Liara is still reluctant, is why I don't think Shepard later getting involved with one of the Mass Effect 2 squadmates is a deal breaker for Liara. If LotSB is played prior to the Suicide Mission Shepard is single, and if it is played after, but Shepard hasn't broken it off with the ME2 LI, Liara is technically the 'other woman.'
As for Tali...I don't think she knows about Liara. If you have Shepard romance Tali in ME2 and Liara in LotSB, I think Tali is under the mistaken impression that she's the only one. Tali will tell Shepard he needs to make a choice before she'll go further for example, if Shepard has also been chatting up Jack or Miranda.