Memorial ring?

Do you know what months their birthdays were? You could add their birthstones into his wedding band

That's a really sweet idea. Not to deter you or anything but when your husband looks down at his ring don't you want him to see you and think of you, not his sisters. There are ways you can incorporate them and their memory like cuffs links or something...that's just my two cents

Engrave their names on the ring

You could do birthstones. But as pointed out the ring is about you and your marriage it's a little odd to have siblings on it. I would reccomend asking a jeweler if you could get a matching bracelet with their birth stones and names engraved on it to match the ring. It'd be a set he could wear.
Because people tend to get family necklaces down the road if you have kids you could get him a necklace to match the set as well. It'd be a good surprise now and later.

I really like the memory cuff links. How many sisters did he have? If he had 2 you could make both in their birthstones.

You know your future husband better than either of us. If I were to do something like this for my FH and incorporate his mom into his ring, I know it would make the ring more special to him. The idea to do something like this is very sweet. If he’s not dealing with any survivor’s remorse or anything I say go with your gut.

View Quoted Comment I agree with this. It’s very sweet that you want to memorialize them, however, your wedding band should represent the two of you and your relationship. You don’t want him to think of his late sisters when he looks at his wedding band. If you do decide to do this, I would talk to him about it first. My brother passed and as sweet as the sentiment is, I wouldn’t want to relive those emotions when I look at my wedding band.

If they were cremated, there are a few companies out there that do different type of things with loved ones ashes. The birthstone idea is a good one as well. You can also do personalized cufflinks with his sister's pictures on them (if there was 2 of them) This is such a sweet gesture. I'm sure he'll love and appreciate whatever you end up doing for him

I'm not sure about the ring, but maybe you could give him something separate on that day? Possibly a locket with their pictures to wear inside his suit on the wedding day?