All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and messenger.
First of all, we sincerely congratulate you for the blessing that Allaah has bestowed on you by making you embrace Islam. This is indeed a great blessing and we hope that Allaah will make it a cause for your happiness in this world and the Hereafter. We advise you to earnestly utilize everything that will help you stay firm in Islam, such as learning beneficial knowledge, performing good deeds and being keen on keeping the company of righteous and pious people.
That sister visiting your wife is a good thing, but the right time should be chosen to avoid difficulty for the household. It would have been more appropriate for your wife to seek your advice in this matter and both of you to agree on an appropriate time, during which that sister could come for a visit, like when you go out of the house, either to work or elsewhere. In any case, this is something that happened in the past and it is more appropriate for you to focus on correcting matters in the future.
The religion of Islam is moderate, and just as it prohibits the causes of temptation, like negligence when it comes to mixing between men and women, it also prohibits being too strict about matters that do not require strictness. The presence of men and women together is permissible provided that they are in separate places and modesty is observed. Indeed a woman may go out to the market, and walk in the way of men, go to the mosque, and so forth. Also, a woman's voice is not ‘Awrah and she may speak while men can hear her. However, she should not speak in a tempting and soft manner. Rather, she should speak good words and in a modest manner. Allaah says (what means): {O wives of the Prophet, you are not like anyone among women. If you fear Allaah, then do not be soft in speech [to men], lest he in whose heart is disease should covet, but speak with appropriate speech.} [Quran 33:32]
In any case, a woman should lower her voice in the presence of men, as this is evidence of her modesty, and there is much good in modesty as the Prophet has said.
For more benefit, please refer to Fatwa 82218.
Allaah Knows best.