I think it's very important for children that age to have manners. But, you know, it needs to be taught otherwise how are they to know?
I have two DSCs (4yo and 7yo) and DH and I have been trying hard for months to get them to say please and thankyou. Their mum is doing the same when they are with her. Our approach has been : 1) first we tried a 3-minute rule, where if they asked for something and didn't say please, they had to wait for 3 minutes and then ask again, nicely the second time. 2) this worked a little, but they were still lacking, and we had told them ENOUGH TIMES over the weeks/months... so now they don't get what they want full stop. E.g. 4yo says "I want my bracelet on" and I say "sorry, you didn't say please, so no bracelet" .. this way they understand the consequences and are slowly but surely learning/remembering. We also make fun of them sometimes, e.g. when they say in a whingy voice "I'm thiiirrrstyyyy..", we say "Oh that's nice, thankyou for letting me know", until they ask in a polite way if we can please get them a drink.
My advice would be - talk it through with your DH, both sit down together with your child and explain why manners are necessary, and set some ground rules.
We only have them 3-4 nights in every fortnight, so even less than you OP, but that's no reason not to discipline. Children would otherwise just get more and more spoiled as time goes on, that'll only make your own life more difficult. My DH does sometimes let them get away with more stuff as he doesn't want them to hate coming to our house if we're nagging all the time, but he definitely doesn't let them run riot either.
"From reading on other forums on here it's also a no no for a non parent to do any form of parenting/discipline" :
I don't think this is true at all - I don't think there's anything wrong with a step-parent doing some disciplining. The children have to know that it's your house they're are living in as well as their father's and that you all have to have some rules to make it a lovely place to live.