She wants to help her fiancé with regard to marriage but her siblings say no

You have done well in your intention to help your fiancé so that he can get married to you. This is something for which you should be thanked, and it is indicative of strong religious commitment and deep wisdom. 

Your helping your fiancé to go ahead with the marriage is helping him with his religious commitment and faith, and this is one of the greatest acts of worship, and is something that is regarded as praiseworthy in Islam. It is one of the best ways of spending one’s money. 

It was narrated that Thawbaan said: When the verse “And those who hoard up gold and silver” [al-Tawbah 9:34] was revealed, we were with the Prophet (peace and blessings of Allaah be upon him) on one of his journeys. One of his companions said: This has been revealed concerning gold and silver; if we knew of any better kind of wealth we would acquire it. He said: “The best of it is a tongue that remembers Allaah, a heart that is grateful and a believing wife who will help him with his faith.” Narrated by al-Tirmidhi (3094) and Ibn Maajah (1856). In Ibn Maajah’s report it says: “who will help one of you with regard to the Hereafter.” The hadeeth was classed as hasan by al-Tirmidhi and as saheeh by al-Albaani in Saheeh al-Tirmidhi. 

Shaykh al-Mubaarakfoori (may Allaah have mercy on him) said: 

“A believing wife who will help him with his faith” means with his religious commitment, by reminding him to pray and fast and to do other acts of worship, and will prevent him from committing zina and other haraam deeds. 

Tuhfat al-Ahwadhi (8/390) 

Undoubtedly marriage helps a man to obey Allaah and keeps him away from haraam things such as looking and listening, and protects him from falling into zina. 

This is in general terms, but you should pay attention to some things, including the following: 

1-The man’s seriousness with regard to marriage and how hard is is striving to meet the requirements of marriage. Because some people depend on the help of others and are not serious about doing what is required of them.

2-If your fiancé is serious and he can afford some of the expenses of marriage but not all of them, there is nothing wrong with your helping him with some of what he needs, such as the wife buying some of the furniture and household equipment, or paying for some of the wedding expenses.

3-The fiancé is a “stranger” (non-mahram) to his fiancée until the marriage contract is done, so during the engagement period it is not permissible for him to be alone with her, shake hands with her or go out with her, rather he is like any other stranger. See question no. 2572.

We ask Allaah to help you both to do that which He loves and which pleases Him, and to bless you with good offspring. 

And Allaah knows best.