When the decision has been made that one of the family members will start piano lessons, (usually the eldest child), the whole family notices. The piano brought into the home will be rather hard to ignore, (regardless if acoustic or electric) as it is a large instrument and takes up space. Furniture will probably be shifted around to make room for it, and any time someone plays (unless using headphones) everybody will hear it.
Assuming the eldest child is the one taking lessons, it would be only natural for the younger siblings to take a very close and deep interest in what their "model for life" is doing. Having an older brother or sister from the day they were born has made the hierarchy in the family very clear. The older sibling has always been a few steps ahead according to the age gap, and has usually been the first to achieve great things in the eyes of their parents, before the other siblings followed along. So if the first born started a musical journey on the piano it should not be surprising to hear the younger siblings asking to have lessons too.
Parents are generally happy to go along with this, especially if they can recognize the positive impact piano playing has had on the older child's life. Another child learning the piano would also mean profiting further from the instrument that is already at home.
Siblings playing the piano could be wonderful. They can support and inspire each other, share new pieces and find ways of playing together, be it duets or improvisations.
However, siblings playing the same instrument may also lead to tension and conflicts. If the older sibling has been taking their time and progressing rather slowy, having an enthusiastic younger brother or sister eager to catch up could be quite disturbing.
To minimize potential rivalry it would be a good idea for the parents to discuss this issue with the teacher before starting lessons. I find it very helpful to use different method books and separate pieces for students from the same family. This makes it harder for them to compare themselves to one another because each method book has its own level system.
As time goes on, maybe not all siblings will want to keep the piano as their main instrument. One of them may prefer to add a second instrument or switch completely to another way of making music, but the foundations they will have learned at the piano will serve them well.
Piano is ideal for learning music notation on both the treble clef or the bass clef and it is the natural instrument for learning the laws of harmony and for understanding music theory.
Therefore it is definitely a good idea for siblings to play the piano and it is an opportunity to give them something they can share and enjoy together at different stages of their lives.
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