3-4 months stay in UK on visitor visa

I live in the UK and over the last 10 or so years I have had relatives from various countries visit me here for duration of up to a month or so on a standard 6-month visitor visa. I know that (notionally at least) if you end up staying in the UK for a rather long period, say 3-4 months, while technically legal, may cause issues in the future when applying for a new visa.

Presently my 18 year old niece, who just finished school, wants to visit us in the UK for 3 months or so. She doesn't intend to work or study; she wants to simply spend time in the UK to improve her English in native speaking country. She would live with us would probably help with childcare occasionally (i.e. babysitting once in a while - but as a family member, without pay). She has no intention of staying in the UK and no reason to want to. She has travelled extensively abroad and has numerous Schengen visas in her passport. In fact, her grandparents are Norwegian citizens and she is considering moving to Norway longer term. Yet, she hasn't visited the UK before.

My question is two-fold. First, is a standard 6-month visitor visa the right approach or is there a better type visa for this purpose? She does not qualify for young migrant programme. Second, if she does apply for a visitor visa, what's the best approach to this?

Updated

Thanks for the replies so far. The nationality is an obvious question. My niece is a Russian citizen living in Russia. She is not eligible for Norwegian citizenship by default, as both her parents are Russian citizens born in Russia. Grandparents are settled Norwegian.