I am 23 years old girl and studying. My family wants me to get married. They are forcing me. But i am adamant at my decision. My dad is sick. He is emotionally blackmailing me to get married. I said no and i am firm at my decision. Is it a sin?
In fact you must say "No" because as long as you remain silent your silence might be regarded as an acceptance:
"A matron should not be given in marriage except after consulting her; and a virgin should not be given in marriage except after her permission." The people asked, "O Allah's Messenger (ﷺ)! How can we know her permission?" He said, "Her silence (indicates her permission).
(See for example in Sahih al-Bukhari)
A woman who has been previously married (Thayyib) has more right to her person than her guardian. And a virgin should also be consulted, and her silence implies her consent. (See for example in Sahih Muslim)
"A lady slave should not be given in marriage until she is consulted, and a virgin should not be given in marriage until her permission is granted." The people said, "How will she express her permission?" The Prophet (ﷺ) said, "By keeping silent (when asked her consent)." (See for example in Sahih al-Bukhari)
Note that as said before if you want to refuse a proposal you must utter it clearly and take the consequences if you came in the situation that you must utter it in front of your sick father or your to be husband you should be nice and careful!
On forceful marriage
Narrated Khansa bint Khidam Al-Ansariya: that her father gave her in marriage when she was a matron and she disliked that marriage. So she went to Allah's Messenger (ﷺ) and he declared that marriage invalid. (Sahih al-Bukhari)
Ibn Hajar al-'Asqalani when commenting this hadith in his fath al-Bary (See here in Arabic) quoted from the sharh of ibn Battal the following:
In the following I'm translating from Arabic language as these translations are of my own you should take them carefully!
قال ابن بطال : ذهب الجمهور إلى بطلان نكاح المكره ، وأجازه الكوفيون قالوا : فلو أكره رجل على تزويج امرأة بعشرة آلاف وكان صداق مثلها ألفا صح النكاح ولزمته الألف وبطل الزائد ، قال : فلما أبطلوا الزائد بالإكراه كان أصل النكاح بالإكراه أيضا باطلا ا هـ
Ibn Battal said: The majority went to (the position of) the invalidity of the forced marriage, while the Kufians allowed it, they said: If a man compels another to marry a woman for ten thousand and the equal dowry is thousand, the marriage is valid and the dowry is obligatory for a thousand and the extra is void. He (ibn Battal) said: When they annulled the excess by coercion, the basis of forced marriage was also void. End quote.
So the majority of scholars consider a forced marriage as void. But not all of them and therefore raising your voice is important here. As once you've done this your waly should think twice before committing a huge sin. As marrying a girl against her announced will is haram based on the ahadith quoted above. Allah also says:
...And do not compel your slave girls to prostitution, if they desire chastity, to seek [thereby] the temporary interests of worldly life. And if someone should compel them, then indeed, Allah is [to them], after their compulsion, Forgiving and Merciful. (24:33)
One shouldn't misuse the power or control one might have over somebody to make them act against their will.
Last but not least
Further have in mind that parents wouldn't make a bad choice for their daughters and therefore it is better to chose the option of dialog and conversation and looking for clarification by making your and your father's position clear and inshaAllah together you may come to a good and suitable solution.
Again I strongly recommend you to check the site for other related posts and especially these two:
In Islamic jurisprudence, the compulsion of a girl to marry is strictly forbidden, so that if a girl marries someone without her consent and only with the compulsion of her father, her marriage is void, and if a child is obtained from this marriage, she is not born lawful. And the son is doubtful and the father is a great sin.  On the other hand, the daughter cannot marry without the father's permission  and must have the father's permission to marry. The reason for this logical rigor in Islam is to find a suitable wife who is both the favorite of the girl and her choice is not based on emotions and is not free of rationality and foresight.
. In this regard, refer to the opinion of the authorities in the answer (site 11885) 10338. . Only in the case of a grown daughter does not require the father's permission. Refer to Rashideh definition (site 7511) 7338.
“No father or mother or any close relation can force his/her children to marry any one against their free will and consent”(Bukhari)
Prophet (PBUH) said: “No female whether a widow or divorcee will be forced to marry any one unless her express and categorical consent has been freely taken and in the same way a woman not previously married can never be forced to marry anyone unless her free consent and permission is taken”(Bukhari)
“If parents force their daughter to marry someone against her wish then the marriage will be void”.(Bukhari)