Obviously you don’t want to hurt your older daughter at the expense of the younger. But in the traditional Jewish community the shidduch window for girls is very narrow, and holding back your younger daughter will not expedite the shidduch opportunities of your older one.
You should have a discussion with your older daughter and tell her how much you admire her generous and kind spirit and her ability to never begrudge others the opportunity for happiness. Give her your berachah that in the merit of her chesed, Hashem will bless her a thousandfold and that her intended mate will appear very soon.
I recognize this is a very difficult decision for you but if you prevent your younger girl from dating she may eventually find herself in the same situation as her sister.
With all that in mind, you might want to discuss this further with a trusted rav or rebbe.
I would also suggest that you tell your daughters to join us at Hineni and I will try to do my best, b’ezrat Hashem, to find them “that right one.”
May you and your husband have the nachas of seeing both of your daughters under the chuppah very soon. And may you see grandchildren and great-grandchildren committed to Torah and mitzvos.