Working remotely for Indian Company while living in Germany

What you are planning is POSSIBLE, but due to the following buerocratic complications DIFFICULT to do:
- If you come on a family reunion visa, this usually includes the right to work (free access to the labour market and also self-employed work) - check yours when you get it!
- The employer must be registered as such and also deduct tax and compulsory social security dues from your salary and forward them, together with their own contributions, to the relevant German authorities. I guess your Indian employer will not be able to do that (unless they have a German office and transfer your headcount there), so you will not be employed.
- Being self-employed requires a whole lot of business registration, accountancy and taxation issues that, unless you are near fluent in (buerocratic) German, you will need to engage professional help for (who of course want fees for their work). There were several threads about this on the forum - locate and read them!
- Being self-employed with only one customer is often seen by the German authorities as attempt to avoid proper tax and social security dues. So to prevent problems (and high costs subsequently), you should write bills to (and get paid by) several clients - not just one former Indian employer.

If you are considering doing this "under the radar", reconsider:
It is not only morally dubious to live here without paying your dues to the host society (world income is taxable when living in Germany, plus compulsory insurances for health, pension and unemployment charges depend on your salary), but also a criminal act that will, in the best case, be fined with an amount at least double of what you saved - and in the worst case can mean imprisonment and deportation. You DON'T want to live in Germany with a constant fear of that!