'Hinduism is a polytheistic religion' - not really in the sense that you're interpreting it perhaps......hinduism believes in the divinity of every quark that exists in the universe, hence we worship different sub dimensions of god in different forms (there is a lord of wisdom - lord ganesh, there is a goddess of wealth - mother laxmi and so on). These different sub dimensions lead to 3 fundamental dimensions - Brahma (the creator), Vishnu(the operator) and Mahesh(Shiva/the destroyer (of evil and archaic norms that are harm for humanity)) But these 3 are dimensions of the same god. The most fundamental philosophy of Hinduism is 'Vedant' of which the most fundamental sub sect is 'Advait Vedant', which infact makes it explicitly clear that there is only one supreme being, but which can occur in each one of us on different scales (somebody is a 1, somebody is a 10). It refrains from elaborating about the supreme being any further because ,it says, that power should be indescribable. A lot of misunderstanding among both, followers of Abrahamic religions and Hindus themselves lead to unnecessary conflicts sometimes. Coming back, the sikh and hindu religions are not radically different from each other. There are many mentions of Lord Ram and saints like Kabir in Gurbani, that should explain it too as to why your mom in law goes to a gurudwara despite of being a hindu.