In English, the present continuous tense (ex. I am going to the doctor) is usually used to talk about something happening NOW. The present tense (I see) is used for things that happen often (ex. I go to the doctor every week). Since seeing your uncle is something that you often do or don't do and it is not something happening right now, you must say "I rarely see my uncle." Saying "I am rarely seeing my uncle" doesn't make sense in English.
Bonus tip: the senses (hearing, seeing, smelling, etc.) are rarely used with the present continuous tense (ex. I am hearing beautiful music) and almost always with the present simple tense (ex. I hear beautiful music).
NOW or in the future, perhaps in our current plans or dreams. "I am taking the test on Wednesday." "We are moving to Canada when we win the lottery." Thanks for the hint on senses.
Im not sure the official explanation, but "I am rarely seeing.." doesn't sound natural in English. It would be more common to say, "I rarely see..."
In English we could also say, 'I don't see my uncle very often.' But not in this case. I seldom use the word 'seldom'.