Hi Tommy,
Welcome to the forum.
In answer to your question, a fairly standard speed that was used for consumer reel to reel is 1.875 ips. It was very common on small machines during the 60's and 70's and was meant for exactly the application that your brother used it for,.. dialog. In fact, the 1/4" tape on 3" reels was used extensively for "audio letters" before the cassette dominated consumer tape use.
My parents gave my sister a little Sony that could play 1.875, 3.75 and 7.5 ips and accepted a 5" reel max.
After I begged and pleaded (and generally made their lives miserable) I was given a smaller Akai that would play 1.875 and 3.75 ips but only used 3" reels. Probably a wise move since I took it apart a couple of weeks later and never got it to work again.
If you wish to hear these tapes, you may be able to find a local library that still has a reel to reel that will play 1.875 ips but be aware, those tapes could very well be acetate and very brittle. If these are important to you, you might be better off getting someone who knows what they're doing to do a digital transfer for you.
There is another alternative to getting them transfered for you. If you were able to play them at 3.75 ips, you can always transfer to digital and slow the speed of the playback in the digital domain. There are numerous editing programs that can do this and you should be able to find one for free.