you are going to have to break up the string, capitalize the first letter, and re-join it. Here is a fairly brute force method

func capitalise(_ string:String) -> String {
    let sentences = string.components(separatedBy: ".")
    var capitalisedSentences:[String] = [String]()
    for sentence in sentences {
        let trim = sentence.trimmingCharacters(in: .whitespaces)
        let capitalisedStr = trim.capitalizingFirstLetter()
        print(capitalisedStr)
        capitalisedSentences.append(capitalisedStr)
    }

    return capitalisedSentences.joined(separator: ". ")
}

trimmingCharacters(in...) removed white space at the beginning and end of a string. The join adds back the full stop and space.

Heres a version using map

func capitaliseFilter(_ string:String) -> String {
    var sentences = string.components(separatedBy: ".")
    sentences = sentences.map {
        $0.trimmingCharacters(in: .whitespaces).capitalizingFirstLetter()
    }
    return sentences.joined(separator: ". ")
}