If I have estranged relatives in the UK, what do I submit in a tourist visa application?

Don't lie, regardless of what a travel agent says: it's an immediate family member, not some distant, tangential relation. Include what information they do have (name, city, email etc.). While your parents may not now plan to connect with them while they're in England, they did attempt contact for some reason, and were ignored.

UKVI is adept at spotting discrepancies and a potential refusal for this (deception) would be a serious ding on their immigration records. Your instincts are spot on: the tour company has little to lose, your parents have much more at stake.

Absolutely mention them. Describe your relationship in one word: "estranged". UKVI will get it.

Lying about it is suicide. The typical punishment is not only refusal to enter, which will then make you ineligible for visa waiver, but also bias future visa applications as well. (In the harsher USA, lying is a lifetime ban.)

Also, from immigration's perspective, they are mainly looking for people who want to overstay and seek employ or collect social assistance. Their first impression of lying about a sister is "she is sneaking you in to hire you illegally under the table". They don't know you.

Of course full examination of the facts and circumstances of your lives would debunk that, but that only works for documents you actually submit, and if you send the wrong stuff it could backfire. If you send documents often faked, then that only further prejudices the matter. You never get a second chance to make a first impression.

Coming from India you can expect your application to have careful scrutiny.

I would give a brief explanatory note describing relatives:

A.N. Other last known address 123 High Street, Anytown AN2 2BC

O.P. Other last known address 456 Another Street, Anothertown, AN3 4BC

No active contact with either