My experience is with a swing gate, but that's not important to the questions you asked.
It somewhat depends on the distance from your MIL's house to the gate. Over 1,000 feet, and all bets are off -- you'll probably need to install a dedicated phone line, like the gates in some gated communities. Up to 1,000 feet, you can install a push button at the end of a low-voltage line buried to the gate. That will open it, but won't solve the intercom problem.
I went with openers from
GTO/Mighty Mule which will make my buddy Wrought'nHarve made at me, because they are not considered to be especially heavy duty. But, I figured I make lots of repairs for the difference in price. We have it set up so both my daughter and SIL and us have the same gates, same controllers and same codes. Folks can come in one gate and go out the other (The drivewways run about 300' on the kids' property and connect to about 800' of driveway on mine).
GTO has come out with some great entry and exit accessories for homeowner prices. The latest one is a combination keypad and intercom that will work for up to 500' wireless and 1,000 feet on low voltage wire. The primary problem with using it as a wired device is that GTO only recommends using their low-voltage wire, and they think it's worth it's weight in gold -- about 50 cents a foot, if I recall, which is $500 for 1,000 feet! The intercom/keypad lists for $399 but I've seen it for $299.
In my case, I put up a $65 keypad outside the gate with programmable codes, and another inside the gate, which has one simple code my guests use to get out. I have assigned different codes to folks like the meter reader, plus my family and friends; they can just come in any time they like. If we're expecting a crowd, like for a party, we include a gate code in the invitation, then set it for the one day only.
For friends who drop by on a casual basis, or strangers who think they would like access for some reason, We're handling it in a very simple way. We all (me, my wife, daughter & SIL) all carry cell phones with us all the time. Nowadays, most anyone who pulls up to the gate and deserves entry also has a cell phone. They can call us no matter where we are, and we can give them a code to punch to get in. If they ar not someone we want to come in again later, we give them a "stranger's" code that we change after they leave. Each keypad can have up to 25 codes, so there are always a few spare codes we can easily remember. Frankly, we don't usually change them. We're not looking for absolute security so much as keeping out kids on 4 wheelers and keeping in the toddlers and dogs.
I haven't taken pictures of our entry and gate, yet, but here's one that shows part of it.