The Frisian pronunciation of Nienke is actually:/'niŋ.kə/ [noted -ed]The pronunciation that is currently listed amounts to /'nin.kə/ in IPA. It is not natively used in Friesland, but rather outside of it, by some (but certainly not all) Dutch speakers. As such, this pronunciation is basically a "dutchization".In order to be sure, I asked native Frisians about this in /r/Frysk and they confirmed it:• https://www.reddit.com/r/Frysk/comments/1fi6o1z/de_nk_in_friese_voornamen_als_nienkenynke/ (in Dutch; replace the 'www' with 'old' if you wish to have a more aesthetically pleasing reading experience)If need be, you can also listen to the audio clips featured on the websites below. However, unless otherwise specified, these were made by a Dutch speaker rather than a Frisian speaker!• https://en.wiktionary.org/wiki/Nienke (in English) • https://nl.wiktionary.org/wiki/Nienke (in Dutch) • https://nl.forvo.com/search/Nienke/nl/ (in Dutch) • https://youtu.be/Gy9DeJcilmE (in Dutch; the name is said at 0:00 and 2:54) • https://youtu.be/keyV9_RQRlE (in Dutch; the name is said at 0:04 and 0:58 and 1:52) • https://youtu.be/dV2XBuAsxZY (in Dutch and Frisian; this bearer is Frisian and the name is said at 0:19 and 0:32)Finally, I would like to note that although Nienke is used in Friesland, it is not exactly native. The authentic Frisian form is Nynke; the spelling Nienke is a "dutchization". It was created in order to make clear how the name should be pronounced. You see, in Dutch, the vowel Y can be pronounced as /i/ and as /ɛi̯/, whereas in Frisian it is always /i/. As a result, the pronunciation of Nynke was ambiguous to Dutch speakers, and so the spelling Nienke was created in order to ensure that the name would always be pronounced the right way.
/'niŋ.kə/ [noted -ed]
The pronunciation that is currently listed amounts to /'nin.kə/ in IPA. It is not natively used in Friesland, but rather outside of it, by some (but certainly not all) Dutch speakers. As such, this pronunciation is basically a "dutchization".
In order to be sure, I asked native Frisians about this in /r/Frysk and they confirmed it:
• https://www.reddit.com/r/Frysk/comments/1fi6o1z/de_nk_in_friese_voornamen_als_nienkenynke/ (in Dutch; replace the 'www' with 'old' if you wish to have a more aesthetically pleasing reading experience)
If need be, you can also listen to the audio clips featured on the websites below. However, unless otherwise specified, these were made by a Dutch speaker rather than a Frisian speaker!
• https://en.wiktionary.org/wiki/Nienke (in English)
• https://nl.wiktionary.org/wiki/Nienke (in Dutch)
• https://nl.forvo.com/search/Nienke/nl/ (in Dutch)
• https://youtu.be/Gy9DeJcilmE (in Dutch; the name is said at 0:00 and 2:54)
• https://youtu.be/keyV9_RQRlE (in Dutch; the name is said at 0:04 and 0:58 and 1:52)
• https://youtu.be/dV2XBuAsxZY (in Dutch and Frisian; this bearer is Frisian and the name is said at 0:19 and 0:32)
Finally, I would like to note that although Nienke is used in Friesland, it is not exactly native. The authentic Frisian form is Nynke; the spelling Nienke is a "dutchization". It was created in order to make clear how the name should be pronounced. You see, in Dutch, the vowel Y can be pronounced as /i/ and as /ɛi̯/, whereas in Frisian it is always /i/. As a result, the pronunciation of Nynke was ambiguous to Dutch speakers, and so the spelling Nienke was created in order to ensure that the name would always be pronounced the right way.