In the old days, marriages were usually arranged by the parents in the Sioux society. The formal request for a bride was made by presenting gifts (the bride price - usually food, blankets, and fine clothing, and later horses) to the bride's parents, who made the decision to accept or reject the marriage proposal for the girl.
Most marriages were arranged this way, although there may have been a few romantic marriages instigated by the young people.
In Sioux culture it was usually the fathers who negotiated the marriage, looking for like minded political alliances, or a social tie that would strengthen the stature of the bride's family in the community, or an accomplished hunter or warrior who would be an asset in providing for and protecting the whole extended family.
While Sioux fathers took the lead and had the final say in such matters, mothers did the steering, and heavily influenced the stance taken by the fathers. Often the father would consider the wishes of a favored daughter, but this wasn't always the case.
If gifts were accepted and the father approved, the girl would have no say in the matter, even if she was opposed to the marriage.
Sioux girls were taught that chastity before marriage was such a virtue, that even an implied loss of it would prevent them from being worthy of praying to the Great Spirit. This was so ingrained into their culture and belief system that they would not even look directly at a member of the opposite sex that was not a family member, and they were given few opportunities to be alone with potential suitors. A good Sioux daughter would never let herself get into such a predicament.
Sioux girls usually married shortly after having their puberty rites, which were held when they reached menses, but males were expected to participate in at least one or more successful war parties or horse raids to prove their valor and courage before they were considered worthy of a wife, so the average Sioux groom was usually quite a bit older, sometimes by as much as 20 years or more.
Older women might also be acquired as wives when a spouse was killed. The brother of the deceased was expected to marry his brother's widow. Occasionally, a divorced person would remarry, but this was rare because it wasn't socially acceptable. Divorce was accepted, but divorced people were expected to remain single for the rest of their lives. Those who did remarry were often ostracized from their band.
Because there were more women than men due to casualties of war and hunting accidents, most Sioux men had two or more wives. Often a man married sisters. This family tie helped to keep bickering and jealousy among the wives to a minimum. A man could have as many wives as he could afford to care for, and more wives meant less work for the women.