First, you need to install the SQLAlchemy library if you haven't already. You can install it using pip:

```bash
pip install SQLAlchemy
```

After that, you can use the following Python code to create the SQLite database with the specified tables and relationships:

```python
from sqlalchemy import create_engine, ForeignKey, Column, Integer, String, Float, DateTime, Text
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship

Base = declarative_base()

class Customer(Base):
    __tablename__ = 'customers'

    id = Column(Integer, primary_key=True)
    name = Column(String)
    credit_limit = Column(Float, nullable=True)
    balance = Column(Float, nullable=True)

    orders = relationship("Order", back_populates="customer")

class Product(Base):
    __tablename__ = 'products'

    id = Column(Integer, primary_key=True)
    name = Column(String)
    unit_price = Column(Float, nullable=True)

    items = relationship("Item", back_populates="product")

class Order(Base):
    __tablename__ = 'orders'

    id = Column(Integer, primary_key=True)
    customer_id = Column(Integer, ForeignKey('customers.id'))
    notes = Column(Text)

    customer = relationship("Customer", back_populates="orders")

    items = relationship("Item", back_populates="order")

class Item(Base):
    __tablename__ = 'items'

    id = Column(Integer, primary_key=True)
    quantity = Column(Integer)
    product_id = Column(Integer, ForeignKey('products.id'))
    unit_price = Column(Float, nullable=True)
    order_id = Column(Integer, ForeignKey('orders.id'))

    product = relationship("Product", back_populates="items")
    order = relationship("Order", back_populates="items")

# Create the SQLite database
engine = create_engine('sqlite:///system/genai/temp/create_db_models.sqlite')
Base.metadata.create_all(engine)
```

Next, you can add some sample data to the `customers` and `products` tables:

```python
from sqlalchemy.orm import sessionmaker

Session = sessionmaker(bind=engine)
session = Session()

# Add sample customers
customer1 = Customer(name='John Doe', credit_limit=1000.0, balance=500.0)
customer2 = Customer(name='Jane Smith', credit_limit=2000.0, balance=1500.0)

# Add sample products
product1 = Product(name='Product A', unit_price=10.0)
product2 = Product(name='Product B', unit_price=20.0)

session.add_all([customer1, customer2, product1, product2])
session.commit()

session.close()
```

This code snippet creates the database schema with the specified tables and relationships, and adds sample data for customers and products. Feel free to adapt it further to meet your specific requirements.