Rather than jealously I see Anne’s thoughts while she plays as sad (of course) but more importantly brutally honest. I think it is a matter of fact to Anne that Frederick deserves all the attention he is receiving from the young women—I think she understands their enthusiasm for him and his enjoyment of their attention. She even understands his high spirits due to their attention, “who could wonder” at it since it is perfectly natural. I don’t think Anne begrudges any them their enjoyment of the evening or each other. As the evening has worn on Anne has realized she has absolutely no power over him, it seems to her not even the memories she has cherished of their engagement has any power over him—I think she already knew this but the evening has really drove it home to her. At the pianoforte she is facing the reality of her situation which is (of course) breaking her heart—IMO that is the reason for Anne’s tears. (;D) As Elinor says in S&S:

Elinor now found the difference between the expectation of an unpleasant event, however certain the mind may be told to consider it, and certainty itself. (Ch. 48)