How can I communicate to my partner that I do not feel comfortable having his/her sister caring for our children anymore, while we are away, without causing a potential conflict between all us?

I think the key here is really communicating both way. You want to be able to listen in a non judgmental way why he/she thinks it's ok while also being able to express why you do not feel comfortable without imposing your point. You want to find a middle ground and by helping your partner see your concerns while also being able to listen to his/her opinion. This will lead some insightful conversation, rather than argument and perhaps you will be able to show each other a point of view the other does not see.

First you must choose the right time to have this discussion-do not force it! Think first to yourself your reasons for feeling this way. Are the children safe? Do you personally not agree with their parenting style or beliefs? Will the children be harmed in anyway while you are away? Imagine if the roles were reversed and your partner approached you with this concern-if their reasoning was vague or "I just don't like them", it's likely to cause conflict. If the children may be in danger, then gently allow them to see your reasoning on their own. Be patient. If, after time, they still do not see it, point out specific moments in which you felt they were at risk. Then, if your partner still does not agree, kindly say that it makes you uncomfortable that they may be at risk and can other options be explored. Offer reasonable alternatives.

Go to your partner and explain yourself. They will listen. Trust and honesty is key in relationships

Perhaps you could sit them down and have discuss this together. If/when the two of you come to an understanding, then try to find a way to inform their sister that their help is no longer needed.

Just be upfront about it. The more you try to delay the situation the more it'll build up and cause a bigger issue for the both of you. Give yourselves space to speak up about the issue. Compromise is key.

You have to go about it with the common factors. What are the pros and cons? What are the downfalls of your significant others' sister caring for your children?

Express your concern, tell them your problem, let your partner give his side of the story, then tell yours. Never raise your voice or throw personal attacks, remain civil and aim to find a solution or compromise.

Maybe you could suggest using another sitter to further socialize the children and give them a different experience with a different caregiver. Different activities, new children, just a new experience. That way it won't seem so much like it's about her but about the kids.

It’s understandable this is a sticky situation. I would be honest with him/her since honesty is the best rule. During my conversation I would use a lot of “I statements” and nicely state the reasons why I don’t feel comfortable leaving the kids there anymore. I would say something to the effect of “while I appreciated her name (keep it in general terms, less personal) I think the kids could also be watched by (fill in the blank) and I would feel more comfortable. Then go on to explain you wouldn’t want that to become an issue between the two of you because as parents there are times when decisions need to be made. I’m not sure hopefully something like this scenario can help you. Good luck