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That said, there seem to be some underlying (and maybe not so underlying) conflicts between you and your daughter-in-law on appropriate "parenting" . Those differences are common, and the simmering disagreement can be highly volatile if not handled with kid gloves.

Whatever you do, be sure to keep your discussions on your differing perspectives out of earshot of your granddaughters; you seem wise enough to already be doing that, but I feel compelled to state it anyway. Many folks assume because 17 month olds cannot verbalize, they cannot understand. Wrong. Discussions (especially heated or confrontational ones) can be jarring and confusing for little ears, especially between people they love. Keep the talk away from the twins.

I'd also use care in not getting your son "in the middle". My hope is that if a sit-down-talk is actually necessary between you and the girls' mom, that it can be done within the common bond of affection, and ultimately wanting what is best for those precious girls.

To try and provide an "answer" for your dilemma, my response (to take or leave) is:
"Grandma's House; Grandma's Rules."

I would hope that if you consistently verbally reinforce those rules with your granddaughters while in your home, their parents will follow suit in support. At the very least, they will not "excuse" or encourage non-compliance with your set boundaries.

Truly, whenever you enter someone's home, whether you are 17 months or 70 years old, common courtesy/etiquette dictates you abide by the host/hostess's requests. To my way of thinking it's never too early to start learning appropriate social behavior!

Of course then the corollary is also true.
At their house, their rules apply (even if their "rules" mean few, or no, rules).

You will need to abstain from trying to police your "Grandma's House" expectations when in your son & daughter-in-law's household.

Be ready. 17 month olds are SO much smarter than people give them credit for.

When you are visiting your granddaughters' home, and the girls exhibit a behavior that is a "no-no" at Grandma's, but "okay" at their own house; they will undoubtedly look to you for a reaction. You may elect to "give a look", but that may be perceived as an uninvited "correction" in your son & daughter-in-law's eyes. The best response, if the girls are seeking one, may be a simple "We're not at Grandma's house." They don't need a lengthy explanation of how you all have different sets of rules. Your daughter-in-law (and son) will hopefully respect that you are acknowledging the supremacy of "house-rules"--- wherever the location.

A little anecdotal aside: My mom (my twins' Grandma) is a preschool teacher. Every year, at least one set of parents gives her the school-starting disclaimer,
"Good luck with ________, he/she is really a handful!" (Of course mistake #1 is saying that in front of the child, giving them a self-fulfilling prophecy to live up to.) Invariably, when the preschool explains, implements, and ultimately enforces their rules/guidelines/behavioral expectations, the predicted "problem child" is amongst the first to cooperate. To my mom's perception, it is typically an example of "people live up to what's expected of them"...and at preschool, they have high expectations. You do, too. You shouldn't have to apologize.

In a quick effort to try and empathize with your daughter-in-law: Since, I don't know the specific dynamics of your family's situation, your D-I-L may have carefully "picked her battles" in regards to the girls' behavior, and has decided certain behaviors, while not ideal, may be excusable in the interest of reinforcing more fully the "serious" violations.

Remembering that 17 month old age (and right on through to 34 months where mine are now) well, there are days when it seems like you are saying nothing but "No!', "Stop!" or "Quit that!" Even when you are attempting to discipline by praising the positive, with two little ones testing every boundary 24/7, the unavoidable correction in a negative vein IS exhausting. Personally, I pick a LOT of battles, but I am sure many would consider me tyrannical.
Every parent is different.

Consistency (as you have SO wisely stated) is IMPERATIVE...whatever those enforceable house rules may be. With two little sets of attentive eyes seeking response, you can bet with every omitted or neglected correction, you are starting over from Ground Zero. With twins it's doubly tough. The "violator" isn't the only one learning from the consequence; the observer sibling is absorbing, too. In reality, that is a true twin benefit. Each correction is doubly taught, each praise for good behavior doubly heard. Of course, it requires more vigilance on part of the "enforcer", but hopefully, it proves worthwhile in the long run.

To use your own words, ".. the girls need to know what to expect here, Mom & Dad or not. ...the rules need to remain the same." I'll second that motion.
"Grandma's house, Grandma's rules." It's location, location, location.

Offer your son & daughter-in-law equal courtesy by deferring to their wishes in their home, and it is my fondest hope all future visits will be joyful ones. It is very important to always bear in mind that the positive, open-hearted relationship you maintain with your son & daughter-in-law will always result in more time with your darling twin granddaughters. Straining that relationship could seriously jeopardize your opportunities to spend with the girls. Do be careful which battles you decide to pick, too! :)

Wishing you ease in cooperation, open communication, and lots of celebration in enjoying your precious grandchildren. :)

Congratulations on your little darlings, Grandma, and they are blessed to have such a caring woman in their corner!!!!

All the best-
Cheryl